I always work roughly on 2 hands on the yearling height and a hand on the 2 year old height, but its a very rough guess. My 2 year ol was about 12.3hh at 11 months old and is now 15hh at 23 months old. The 3 year old was about 12.1/2hh at 2yrs 3 months, and at 3 yrs 2months is now scraping 14hands and looking like hes got a bit to go yet!
I expect the 2 yr old to finish off at over 16hands. The string test pegs him at 16.2hh. I'm hoping the 3 yr old scrapes 14.2hh by the time hes 5 or 6.
I've also been told the general rule of thumb of 2 hands onto the yearling height or 1 hand onto the 2 year old height. My 2 year olds now stand at 16.1 (expected originally to make 16.2, now thinking maybe upto 17hh) and about 14.1, (expected to make 15.1-15.2) At 12 months old the bigger one was 14.2, and the smaller was about 13.2. So for your's i'd say the 2 year old to make about 14hh plus a possible inch or 2?! and the yearling to make 14.3 poss a bit more again? it really is just general guessing, some horses seem to do all their growing early on, others seemt to grow slowly but for longer etc...
Just remember they can go against the rules, at 18 months my tb x was 15.3hh and at 4 she is 16hh . But i would agree that yours should make around 14hh give or take a couple of inches .