Using data and calculations from QuickLoad software, a .243 will have 81.5% of the recoil of a .25-06, given the following assumptions: both cartridges are loaded to "nominal maximum pressures", both rifles weigh the same, both use the powder giving the highest velocity at nominal maximum pressure, and the
25-06 is loaded with a 100 grain bullet and the .243 with a 90 grain bullet. If any of the parameters (weight of bullet, rifle or powder or velocity of bullet) are changed then the percentage difference in recoil would also change.