I'm looking for a little advice regarding proper boot flex in a racing boot for my soon to be 14yro son. He's on the small side at 5'2" 115, but he's definitely decided to start growing having put on 2-3" and 20lbs since this time last year. I did a quick shell fit and the 23.5 Atomic RT sti (70 flex) that were new last year shows maybe 5mm of room behind the heel, so it's seems he's in need of the next size up, likely a 24-24.5. He's a very strong skier, had close to 100 days/year on snow last year, and is entering his 5th season of ski racing.
It seems he's been in 70 Flex boots for at least 4yrs in a row, but with the added weight and strength, it's probably time to stiffen the boots up a bit. But by how much? up to a 90? 100? 110? do you guys use any formula based on weight or? I know putting him into something too stiff would be detrimental, but I also don't want to put him into something too soft either. I have an email in to his coach for feedback too (coach wants him skiing on last years wm's FIS regulation skis, 183 GS, 158SL) but am still waiting to hear back. We also have an excellent local boot fitter to work with. I've kind of resigned to the fact that his feet are still growing and I may be getting new boots again for him next year, but if it was possible to get 2 yrs....