WIY- I had ther opportunity to quickly review all of R’Moshe’s teshuvos on mechitza (in chelek 1, OH”CH,chelek 3 OH”CH and chelek 4 OH”CH”) and ,indeed you were right in insisting that R”Mmoshe zz’l requires sixty inches and, preferably,even higher so as not to see the women. So, you are right as far as R’Moshe zz’l. Although, clearly, there are and were Poskim who allowed ten “tefochim”. There are a number of questions that R’Moshe himself tries to answer in his latter teshuvos. The primary question I would have is the fact ,that if you say that a mechitzah is up to the shoulders, how can it be 18 tefochim? A man’s height-as per mishneh and gemoro- is three amos (see mikveh)which is 18 tefochim so, ‘baal korcho’, up to the shoulders is significantly less? R’Moshe (in chelek 3) tries to answer but I did not fully understand his answer.
R”Moshe insists that it is ‘mideoraisa” and he has a fairly complicated way of deducing this from gemoro sukkos. There are many problems with this approach but this forum is too restricted to go into detail.