If/Then Process

I know that would be really great. But it is over my old head.
I'm trying to calculate a group of cell's from a starting number and incrementing upward on an option of either percent (%) or a numerical number like .01 grains.
I think a box with 2 buttons with the option buttons to instruct Excel either 1). Percent, or 2) incremental amount.

I think an If/Then statement would say is "If Percent Button is chosen calculate (#*%)+#. In other words =C4*C3)+C4 OR
If Incremental is chosen Then calculate (#*#)+#. C4+D3.

With data validation in C1 (source: Percentage, Amount), C5=IF(C1="Percentage",C3*(1+C4/100),C3+C4)