well.. add as much as you want, some people use 50/50 perlite and compost, some only use a little perlite... it varies.. my advice would be to measure out 3 different ratios of compost and perlite, record which one does best for that strain, and use the mixture you most like for your 'serious grow' when summer comes around then you can grow the other 3 much better, and it will keep you interested... good luck..

p.s. watch the temps at night if your growing in a greenhouse...