Projecting forward

Hello

Sorry if this is not the appropriate forum. How would one calculate the reduction on a return for a regular savings amount over a given time frame with a % ongoing charge and absolute flat charge deducted over time? I.E. assume £100 saved each month over 10 years and say a monthly £2.00 charge, 0.5% of the annual accreting savings value (assume the annual charge is deducted each anniversary). I am ok with the annual % charge in isolation but the part I struggle with is the saving amount and £2.00 flat charge since these as a % of the growing amount over time will obviously become less and less, lets assume there is a 7% growth year on year before any charges. What I am looking at is the reduction these charges will have n the 7%. Many thanks in advance, Darren.