option button help please

Good afternoon all,

I am having a little trouble trying to figure out how to get an option button working ..
I have attached a sample of what I have so far and what I would like to do is:-

I have put 3 option buttons on my user form ... I would like them to add 25% and 50% to the textbox results that are selected..

You select Index material and description .. this will give 30 prices on the form ... I then wish to have standard (by default) and the other 2 options to be selected as needed

Many Thanks in advance