Mixing soil with perlite questions

I'm going to be using Coast of Maine Stonington blend for my first indoor grow. I've read that its a pretty dense soil and should be mixed with perlite to help with drainage. Is there anything I should be looking for when I pick up some perlite? What percentage mix should I be using between that and the soil? This will be my first indoor grow so hoping to get it right (or at least as close to right as I can...)

I like the big chunky perlite and I mix it at 25%

I bought the big chunks as well, but all the soil I buy has the small ones in it.... so?
Mix 30% or so, you want it to drain well in a reasonable amount of time.

I like to use pumice vs perlite for many reasons. Same percentages as perlite.