Quite right! And, as I realised after the first three minutes of trying to check independently, the money goes in weekly and is compounded monthly, so that won't fit easily unless I take an average monthly amount, which I may as well do, actually ...

Back to sherryillk's formula! It wouldn't work for me for m>c, but maybe it will if m=c!

I knew I would regret not practicing my maths as much as my French and Italian one day!