Don't know if this is true but during my training we were told the VNAV kit developed for the 757/767 was derived from the Space Shuttle Orbiter's descent nav. system. Interesting seeing as the Shuttle uses a M1.0+ 21Degrees slope.
Although I have to say on the basic 757 "it" invariably panics just after TOD and demands speedbrake. However if you take no heed it generally sorts itself out without having to resort to the "rumble Handle" after a few mintutes.
Very clever pieces of kit but because I'M the one who programs it I'll never fully trust the "Pink line".