Hey Paul,
I don't know that there is a "rule" regarding length of taper. I will lower the entire background behind the cheekpiece to make true bas-relief carving because there is not enough room to taper into the carving. The rest of the carved areas I will run a taper out to about 1 1/4" leaving the elements about 1/32" or so high. You want your taper long enough so the carving looks like bas-relief instead of the incised relief it actually is. The prouder the carving stands, the longer the taper must be. Be sure your tapers don't take a dive at the base of the elements. I look at areas between carvings where the tapers run in opposite directions ie, between the lock panel teardrop and the wrist carving, in profile to see if the uncarved area looks " raised". If it does, I scrape it lower.