Fraz, you need to know two things to figure it out exactly. One is the distance between your eye and the front bead when you stand in firing position. The other is target distance.
The formula is simple. It is Target Distance in inches divided by bead/eye distance = x times y, where x is the distance multiplier and y is the distance you move your comb.
For example, there are 45" between my eye and my bead. Singles targets are hit about 32 yards from the shooter. 32 yards * 36" = 1152". 1152" (target distance in inches) / 45 (my eye/bead distance) = 25.6 (distance multiplier) * 0.125" (comb shift) = 3.2". So your POI moves 3.2" @ 32 yards for a 1/8" comb shift. It would move 6.4" for a 1/4" comb shift (25.6 * .25").
Fraz- The method described by zzt is correct. But, such a calculation may not be useful unless you know exactly how far under the targets you are shooting now.
Most of us take a different approach. If we are shooting too low, we raise the comb up 1/8 of an inch and see if that helps. You will need to shoot a lot of targets to determine if the 1/8 inch change made a positive difference. I do make some stock changes now and then. I know I probably shouldn't, but I do. I try to shoot around 1000 registered birds before I decide if the change was good. Once in a while, I only have to shoot 50 birds to realize that the change was bad.
washandwear, Fraz is talking about offsetting the comb. That is different than cast. The calculations are the same no matter what you move. Just remember to pay attention to direction. Changes to the comb move the POI in the same direction the comb was moved. Changes to the bead move POI in the opposite direction.
Also keep in mind that when shooting targets to check the effects of comb movement on POI (and target breaking effectiveness) that you must also consider if head (and eye) movement during swings might be involved in the POI change.
It is movement of the eye (relative to the rib) as a result of comb adjustment that changes the POI. In changing the POI with comb adjustment, it is possible that head movement is at least part of the problem. If it is, an aspect of shooting form (possibly the gun mount you are using), possibly in combination with ill-fitting stock dimensions may be effecting POI.
If that is the case, changing the comb position may just be treating a symptom of a different problem.