Some folks do 40/60 perlite to soil or 50/50, hell I've seen folks on here doing 100% perlite. I usually do 30/70 perlite to soil and that seems like plenty to keep the soil soft. Don't forget to rinse the perlite before you add it to the soil or you'll mess up your roots.
I would shoot for about 25% perlite if you're mixing with straight soil. Most people will say 20-35%. I put 25% in mine and im happy so far (first grow). I've used it in a couple house plants and <20 didn't help as much as I'd like while the one i put maybe 45% in drained much too fast for my liking. Miracle gro perlite has nutes id stay away from for my ladies!
So if I mixed 4 gal of this soil I have with I gal of perlite. That makes 5 gal total will that be good enough. The soil I have is airy but not like the soil I used to use