Modify Calculated Forms

Hello, I'm wanting to modify a field that is calculated

The fields are thus;

Calculated Field-"strain"

It calculates from  "vig_type" &"vig_bonus"

Also, if it isn't too hard, I want it to automatically add the difference if either of those is changed, from the current value, but, I don't know java well, so I'm not sure if this is possible, but if not, then just re calculate if those 2 are changed.

Thanks.