Blender Git Commit Log

use the SUM driver type to avoid adding all values on each bone.

also avoids hitting the 255 string limit if you want to add 100's of values. eg.
>>> b05/max(0.001, [globals().update({"LOCALS":locals(), "ADD":float.__add__, "reduce":__import__("functools").reduce}), 0.0][1], max((, [LOCALS["b%.2d" % (i+1)] for i in range(5)])))

Since this more simple expression reaches the limit fairly quick...
>>> b05/max(0.001,b01+b02+b03+b04+b05)