Hi bean, The talk on crossover led me to take a look at "Handling the Big Jets", by D.P. Davies, once Chief Test Pilot for the ARB. The book is probably long out of print, my own copy being one I was given in '75. There is a graph, which I don't think I can insert, of descent speeds. He is using Mmo 0.88, Vmo 330. The crossover on his graph is at 31,000', with "coffin corner" at 50,000.
Having said that, I'll shut up. My turbo-prop experience is C-130 and F-27, so I have nothing further of value to add to this thread. However, I'll continue to follow you all; always something to learn.