I did some searching on this, couldn't find anything aside from "HP in white", but based on how we've seen latent for Parade Gorget working, I decided to do some testing, and figured maybe others can test too to pull some exact figures.
In my test, I did WAR/RDM (has same base HP as /NIN), and used Venom potions to lower my HP, and test the effect with Stoneskin using my leggings (wish I had cap or hands to test easier instead of with spell recast):
My Base HP is 1139, with +20 HP from food. Geared, which is how I tested, is 1235 +20 HP, for a total of 1255 when testing.
At 894 HP, the latent was active, and 893 HP it didn't work.
I have +86HP from armor [items that your character would 'blink' from when equiped/unequiped] (Turban, Dusk Gloves +1, Unicorn Leggings +1) +10HP from accaccessories (Toreador's Ring), 20 food.
By what I can tell, the latent seems to be:
HP > 75% of [Base HP+ (HP from armor)-HP from the Unicorn piece].
In this case, according to the formula, (1139+86-34), or 1191. 1191 -25% (297.75)= 893.25, so when HP = or > 893.25 (or 894 because HP is an integer), the latent is active.
It seems to check out with my setup, and was hoping some other people could confirm or refute this please.