percentage calculation from cumulative totals

Hi there
I need a formula for finding the performance in a percentage against a target saving and a cumulative total. I've tried various ones but have failed miserably. Everytime I think I've cracked it the percentage keeps going down even when I hit target or go over?!?

Help please

Attached is a sample of what is needed, it will probably be simple for you guys but it has me stumped...