at beginning of move set feedrate to average of last and current feedrate.  do move with that feedrate.
then set the feedrate to the current feedrate.
if it's just a feedrate change, then the feedrate will become the feedrate.
if there is no feedrate in current move, feedrate will be last feedrate
if there is a move and a new feedrate then the move will take however long the average feedrate would take.

estimation is a bit more optimistic than pessimistic.  however it seems to be a lot more accurate.