[theory] good chances for call..

[Karn]

situation smth like this:
Hero(BB)(7BB)
SB - (3BB)
Button - (10BB)
CO - (12BB)

CO folds, Button folds, SB is allin. U have good chances to call on any 2:
u need to bet 2.5 BB in 4.5 BB pot. But if u lose, next hand u on SB and after postin a blind your stack will 4BB, and u will have very small fold equaty against BB...
lets imagine that u exactly know the hand of SB. What chances do u need for your hand to call? 50/50? 40/60? 60/40?

How will your decision change if there are 5 players instead of 4, and 5th has 10BB?