If you are just interested in knowing if a calculator is allowed you would have to check you local rules. I would think in most cases the answer would be no.
If you really need to calculate the spot on the rail I would suggest you learn the off the rail aiming system. This is sometimes called spot on the wall system. You just use your aiming system for whole diamonds then you use the spot on the wall. I know this is an over simplification but if you do a little research I am sure you can get a thorough explanation of the system. Probably Dr Dave or Bob Jewitt have some articles explaining this in detail.
If you really need to calculate the spot on the rail I would suggest you learn the off the rail aiming system. This is sometimes called spot on the wall system. You just use your aiming system for whole diamonds then you use the spot on the wall. I know this is an over simplification but if you do a little research I am sure you can get a thorough explanation of the system. Probably Dr Dave or Bob Jewitt have some articles explaining this in detail.