Lots of variables in growth rate, food amount & frequency, water changes, internal parasite load, dominance in the group pecking order. IME they grow fairly fast from 2 to 4 inches, moderately to 6 inches then much more slowly. The largest I've ever grown were ~7+ (SL, + tail ) inches after maybe 8 years starting with 1.5 inch clowns but some were a bit smaller.
And from what I've seen, the likelihood is the entire pod will never reach that size. there seems to usually be a pecking order, as fishorama said, that becomes obvious as some grow large and others stay small. You'll usually end up with a variance or several inches.
20% is probably not enough, at least after your 2 inchers begin to grow. That said, I never started out in a 240g, more like a 4 ft 55-75g. Keep an eye on shoal dynamics & nitrate level feeding 3x. I'm a generous feeder & 2x is the most I ever did, you want good growth but not obesity :shakehead: or high nitrates (over say 20ppm). Of course we'll need pics...please!. I had to give up my latest batch of clowns & will be starting overagain... soon, I hope.