bounce flash

[NEPats37]

i am shooting a pretty low light concert and am planning on bouncing the flash using a index card mounted to the flash. i wanted to know how much I should adjust the f-stop by when bouncing the flash. the number on the back of my flash is just for direct flash. is it 1 stop 2 stops?
thanks

 

[Hertz van Rental]

Is it a comp controlled flash or a manual?
If it's automatic then it will compensate for you.
If it's manual then the formula for finding the approximate f-stop is:

f-number = 0.7 x Guide Number/d1 + d2

where d1 = flash to bounce board distance and d2 = bounce board to subject distance.