Maths isn't and never has been my bestsest subject. 
I've recently become registered to the VAT club, on the flat rate scheme, and accordingly I have to pay 7.5% of the VAT that I collect, only for the first year I only pay 6.5 %.
The downside is that I don't claim any VAT back unless it's a large purchase.
I'm still going to be slightly 'up' at the end of the year though I estimate.
Anyway, to save me ringing the VAT office up, how the hell, and what is the formula for working out 6.5% of 15.0%. as I have to do my first return for the end of this month? 
It'll be another calculation in January when the VAT goes back up to 17.5%, but they haven't yet told me what I'm going to have to pay...problee 9% I'd have thought.
Okay... if I understand correctly...

if the gross amount you bill is ?1,000
Divide by 115 and times 100 to find the VAT... ie, the net amount is ?869.56 so the 15% VAT is ?1,000 minus ?869.56 or ?130.44
Then, you just need to find 6.5% of ?130.44 - correct?
?130.44 divided by 100 and times 6.5 makes ?8.48 (rounded up).
And I see why you get to pocket the rest as you stated you can't claim VAT back on your purchases...