Valentine's Event shiny chance:

The shiny chance percentage formula is (C/X)² * 100, where C is the number of cards sent. X is the number of cards needed for a guaranteed shiny, and has been different each event. Though there's only been two of them with the new formula: Enamorus needed 150 cards to be a guaranteed shiny, and Cupidfly needed 100. So X likely depends on the rarity of the Pokémon.

 

To find X:

X = C / (S / 100)^0.5

 

S is the shiny chance percentage displayed on the event page, and C is the number of cards you've sent so far.