I'm not sure how useful this may be, but for those who are going to limbus for your +1 pieces, I tabulated where upgrade pieces commonly drop from. The drop data is from otherwiki, since bgwiki doesnt have anything. I'm not entirely sure how accurate it is, but its better than nothing so far, since everything else shows a full list of what can drop, not the common drop of every floor.
Code:
Common Areas (15% chance or higher)
Apollyon SE NE SW NW
1 Warrior 4f 3f n/a 1f
2 Monk 3f n/a 4f 4f
3 White Mage 4f 1f n/a n/a
4 Black Mage 4f 2f n/a n/a
5 Red Mage 1f n/a 4f n/a
6 Thief 1f n/a 4f 3/4f
7 Paladin 2f n/a 4f 4f
8 Dark Knight n/a 4f 1f 2f
9 Beastmaster 2f n/a 4f n/a
10 Bard n/a 4f 1f 2/4f
11 Ranger n/a 4f n/a 4f
12 Samurai 4f 1f n/a 1/4f
13 Ninja n/a 4f 3f 2/4f
14 Dragoon 4f 2f n/a 1f
15 Summoner n/a 4f 2f 2/4f
16 Blue Mage 4f 2f n/a 1f
17 Corsair n/a 4f 2f n/a
18 Puppetmaster 2f n/a 4f 4f
19 Dancer 1f n/a 4f n/a
20 Scholar 4f 1f n/a n/a
Temenos West North East Central (each area drops 1 job it seems)
1 Warrior 1/5 n/a n/a 1st
2 Monk n/a 2/3 n/a 3rd
3 White Mage 6f 3/4/5 n/a 3rd
4 Black Mage n/a n/a 2/6/7 2nd
5 Red Mage 2/6 n/a n/a 1st
6 Thief n/a 1457 4f 3rd
7 Paladin n/a n/a 1/2/3 2nd
8 Dark Knight n/a n/a 4/5 2nd
9 Beastmaster n/a n/a 1/6/7 2nd
10 Bard 1/2/3 n/a n/a 1st
11 Ranger n/a 6f 1/4/5 2nd
12 Samurai 2/4/6 n/a n/a 1st
13 Ninja 6f 1/2/6 n/a 3rd
14 Dragoon n/a 1267 n/a 3rd
15 Summoner 1567 n/a n/a 1st
16 Blue Mage 1357 4f n/a 1st
17 Corsair n/a 1347 n/a 3rd
18 Puppetmaster n/a n/a 1347 2nd
19 Dancer n/a 3f 5/6 2nd
20 Scholar 3f 5f n/a 3rd