Nested IF function or VLOOKUP for picking a value and adding to existing value

Hi,

I am building a spreadsheet to track my training program and have got stuck. The number of reps performed on the final set of the previous week's session decides what weight to move onto next week. I am struggling to find a way to update the weight to be used depending on the value picked from a drop-down box.

I have attached a sample version below with extra detail on it.

Also if you would like this program let me know and I will email you the final copy.

Thanks in advance
Dexter