Its trial and error really, as all dogs vary so much as to how food they need.
Do you have access to scales to weigh your dogs? If not then do it by sight/feel. If you feel happier cutting their food back a bit to offset their treats then maybe take 20g of a day. Then over a couple of weeks see if they gain weight drop the amount a litter, if they lose weight and they don't need to up it a little and if they stay as they are then keep it the same. There isn't an exact science unfortunately.