its hard to know how many letters a kid will take because you will (unless it recently changed) get a positive message to ALL the letters and calls you send in a given cycle, no matter how many, unless you are already over the limit (or maybe at the limit?). you don't actually get credit for them all though (i tested that). i've never taken the time to figure out the letter/call counts after they got rid of international unlimited letters (nooooo, although i've honestly had to send 40k in letters before, and that was incredibly brutal - almost 300 sets of letters - so im actually not that sad about it) but certainly if 40/40 doesn't work, don't keep throwing more on, you probably got 0% effort for a lot of those.