NONE OF THIS APPLIES TO FISHING
Skillups occur on both main and subskills independently.
Skillups occur even if if your enhanced skill (natural + synth support + synth items) exceeds the cap.
For any craft above your natural craft skill below 50:
6 or farther from cap: somewhere between 66% and 75% chance to skill-up on a successful synth
5 or less from cap: somewhere between 66% and 75% chance to skill-up on any synth, broken or not
For any craft above your natural craft skill 50 or above:
6 or farther from cap: somewhere between 25% and 33% chance to skill-up on a successful synth
5 or less from cap: somewhere between 25% and 33% chance to skill-up on any synth, broken or not
Skillups on breaks will not occur if your natural skill is beyond 5 levels from the cap, even if your enhanced skill is within 5 of the cap.
Above 60, skillup increments are always 0.1.
Below 60, skillup increments can be 0.1 or 0.2.
If your natural skill is 6* or more below the recipe cap, skillup increments can also be 0.3.
If your natural skill is 12** or more below the recipe cap, skillup increments can also be 0.4
If your adjusted skill is 16 or more below the recipe cap, you will be unable to attempt synthesis.
It is possible to gain 0.2 (or more) skillup even if you are 0.1 from the recipe cap. However, it is not possible to gain 0.2 (or more) skillup if you are 0.1 from an active guild-enforced craftskill cap. For example, making iron chains at 29.9--if you have "broken" the 30 cap already by turning in a test item, it is possible to get 0.2 and end at 30.1. If you have not broken this cap, the maximum skillup increment will be 0.1, which will cap your skill at 30.0.
* Needs more verification
** Might be possible at 11; definitely possible at 12
For multicraft items, break rates are cumulative with each involved craft. That means if an item has 3 crafts involved, find the break rate for each craft and sum them up.
Thus, for each craftskill involved:
If your enhanced skill is at or above the recipe cap, the break rate is ~5%.
If your enhanced skill is below the recipe, cap, the break rate increases by approximately 10% per level below the cap. *
* Needs more data
Item loss rate on break is approximately 50%. Moghancement: crystal for the appropriate crystal lowers this by around 5-10%. Moglification probably has a similar effect while still providing +1 skill.
Certain crafts have nearly-guaranteed loss components. Particularly cursed item crafts and key unstackable components for "elite" crafts, such as VClaw, Elemental Beads, Damascus Ingot, Damascene, Raxa. These items can be preserved, but the rate is extremely low.
After the game has decided your craft attempt was a success, it will roll for HQ. Note: HQ calculation is performed after the check for breakage. Thus, higher-break-rate items (multicraft items particularly) will naturally have slightly lower overall HQ rate, due to a smaller pool of potential HQs.
To calculate the HQ tier of an item, compare your enhanced skill versus the craft cap. If the difference is 51+, the rate is T3. If the difference is 31-50, T2. If the difference is 11-30, T1. If the difference is 0-10, T0.
Theoretically there has also been proposed a "tier -1" for enhanced skill below the cap, but not much data on HQ rate has been provided except that it seems to be much smaller than T0.
For multicraft items, the HQ rate is based on the lowest gap between enhanced skill and recipe cap. That is, if you have a multicraft item where you are T1 on 2 craftskills and T0 on the 3rd, the entire item will have a T0 HQ rate.
T0 rate seems to be somewhere between 1 and 3%. Some people predict 1/64.
T1 rate seems to be somewhere between 5 and 10%. Some people predict 1/16
T2 rate is about 25%. (1/4)
T3 rate is about 50%. (1/2)
Once the game has decided that your craft was HQ, it will determine the HQ strata.
Craftskill plays absolutely no bearing on this calculation.
The chance of the HQ being a HQ1 is 12/16
The chance of the HQ being a HQ2 is 3/16
The chance of the HQ being a HQ3 is 1/16
For crafts with only 1 HQ, this check is irrelevant and may not actually be performed.
For crafts with no HQ3 but a HQ2, the ratio may be slightly different depending on the implementation SE used.
Unpaid support is +1.
Paid support is +3.
Belt support is +3.
You cannot have support from multiple guilds simultaneously.
Each craft has 4 main items obtained from GP which provide permanent assistance.
The Apron and the Accessory each give as stated: +1 skill.
Cooking also has a lv57 NIN katana which gives +3 skill called Hocho.
Some crafts now have a "latent +1" item as well. This latent seems to be active if your natural skill is below 60 and you have no synthesis support from the guild. IE, trash.
Each guild has 2 furniture items. Having either one in your moghouse can provide Moghancement: <craft>. This will give you an additional +1 enhancement, which is always active as long as you have the Moghancement key item. Having both can provide Moglification: <craft>, which gives you the +1 enhancement as well as reduces item-loss for crafts in that field.
Elemental energy (faint/powerful/overwhelming) has no effect on Moghancement: <craft> or Moghancement: <crystal> and probably not Moglification: <craft> either.
Currently the maximum attainable craft level is 100. This was last changed in December 2003. Certain items have a craft cap over 100.
On a single character, it is impossible to fully specialize in multiple crafts. Each character has a pool of 40 points he can allocate to his crafts above 60. So for example, one could be 100 in a craft and 60 in the rest. Or, 80/80 and 60 in the rest. Or 75 in 2 crafts, 70 in a third, and so-on.
If a character gains a skillup which would put him above 40 allocated points, points will be deducted from the other highest craftskill above 60. Since skillups past 60 are never more than 0.1, this will always happen in increments of 0.1.
Moonphase/Day of week/direction
Absolutely zero solid data supports any theories about how to improve HQ rates/break rates/skillup rates.
Don't make me sic Sath on you.