This is more of a "doop" than a "eurika" but it finally occurred to me that my luggage scale can quite easily be used to tell me exactly how much filament is left on the end of the reel. Like I said, doop, not eurika!
Weigh the reel of filament, subtract the weight of the spool, subtract the weight of the filament that is unusable (Ultimaker specific to it's feeder), compare to the grams needed for the print from the slicer.
This is a particularly accurate measurement now that I have standardized on one size of filament and in most cases to a single manufacturer of filament. Historically I had a number of different spool sizes and hence weights.
Hey, I did say this was a doop not a eureka!