As a rough guide add a hand between 1 and 2 then anything up to another hand up until maturity. So possibly up to 16.3. How big is Mill Law? My now 2 year old grew a hand between 1 and 2 and is now 15.1 i'm expecting her to make 16.hh.
When small hunters were measured we had one measure out at a County show in May as he was 14.3 and a bit as a yearling and had to go into the big boys class.
Although they no longer measure I still use that as a guide - so if your youngster is a yearling and not over 14.3 then maybe you will stay under the 158 mark - lucky you.
Saying the above it does make a difference on how the horse is bred and how well it has been done. So a youngster with little TB blood which has not been produced will take longer to reach maturity IMO