RENURPP
I totally agree with your maths, but 470 to 320kts is not instantaneous, and with a 100kt tailwind 570 to 320 less so) hence my numbers. With the given proviso that once at speed, recalculate the speed required to make the waypoint.
But in reality, the FMC RTA function works fine for me (Level forecast winds entered) . And considering the speed reduction requests are normally made when it all starts to get busy, the FMC option gives me more time for other things.
But I am first to admit that there are many ways to pluck a duck and respect each to their own.
Cheers