Commission Generated Question

Despite (apparently) being in the top 0.5% of Betfair's customers, I guess I am sometimes a little slow. When I first read about this new Premium Charge I assumed that if I had a gross profit of £1,000 in a week, and I'd already paid £75 in commission, then the Premium Charge would be £125 making my total commission £200, or 20%.

But this is not so - Betfair use the 'Commission Generated' to calculate what I will pay, not Commission Paid, so how the heck do I work out what my Commission Generated total is?

I guess I shall be making another call to find out how I can work out this number. I hate the idea of not knowing just exactly what hit I am going to take each Wednesday, and it certainly wouldn't be right to charge me a fee that I have no way of verifying is correct.