PTR will tell you how various areas compare to the average stats for the top winning players at your level. It will not tell you how you stack up if you intentionally play LAG or nit styles since more players strive toward TAG styles.
If you are truly playing a nit style, then you should be more aggressive than your opponents because you're only playing with premium hands. But tighter than 75% doesn't necessarily mean that you are a nit; most players at the lower levels play waaay too loose.
At various points PTR lists the top winners at various levels. Search that out for your level and compare the styles of the winners. See if you can find one that plays similarly preflop to you, and then compare the rest of your ratings to his alone. Maybe also look for someone one level higher who plays a similar game to yours for comparison.
And, 20/8 is not optimal. Standard ratio should be in the 3/2 or 4/3 range, or tighter. If your VPIP is 20, then PFR should be 14-15+. If you play 30, then PFR should be 20-22+. Looking the other way, if you only raise 8%, then at most you should be only playing 12%.