Numbers listed on the card are already included in the item stats. So card damage shows 100, Text includes 50% bonus is going to do 100 damage.
Critical hits are the only number you actually need to go and manually calculate the multiplier for.
The most common occurrence of damage on an item card is generally on red text guns. This is because the Red Text has a special effect, for example let’s talk about the Monocle:
The reason the card lists -24% weapon damage is because this gun is based on a purple Jakobs sniper, Purple Jakobs snipers as most know commonly have a card damage in excess of 2k, the Monocle has a max card damage of about 1600. To create the monocle Gearbox subtracted 24% damage and in return gave it a hidden 250% critical hit bonus.
A second example is the Maggie
(Image from Moze’s Top Gear)
The Maggie’s red text effect is the number of bullets it shoots per trigger pull. The purple gun it is based on however is a Purple Marshal (I might be wrong about which gun it’s based on but the idea holds). The Marshal has a damage of about 1000 and only fires 1 bullet. Can you imagine the power of 1000 x 6? Gearbox didn’t want this so in return for the extra bullets they halved the damage each bullet does on the Maggie, this is why all Maggie’s have -56% weapon damage on the card.