using validation box to implement different formulas with IF function

i'm required to calculate share returns using 2 formulas Chang: r=(p(t)-p(t-1))/p(t-1) and log return: r= ln(p(t)/p(t-1)) rather than enter 2 formula in 2 columns, i need to use a validation box and IF function. eg. if 'change is selected in the validation box, it should apply the 1st formula automatically. the column function uses an IF function. I've managed to set up the validation box with 'change' and 'log' what should i do with the log function?
thx

Probably i can't help you enough with your question.

But I think,

You get better help if you post an Excel-file (example of your workbook), without confidential information.

Make sure the workbook demonstrates your desired results if possible, or just highlight the cells you're trying to fix.

Use BEFORE/AFTER sheets if that helps make it clearer.