How I can calculate the margin requirement when given in a percentage?

You need to calculate your overall exposure of your position which is done by multiplying the instrument price by the quantity.

The next step is to divide the exposure number by the leverage factor.

If you have 10 CFD's of a value of GBP 500, the exposure is GBP 5,000.

If the leverage is 10:1 then the margin required would be GBP 5,000 divided by 10 equals a margin requirement of GBP 500.