try breaking it down by hand first, twist, break, rip etc
then cut into small squares, try for less than 1 x 1 cm, though some peices are just unruly
then grind in small batches, cant really give a number hear as per gram but keep it small. my grinder has a fill to here for spice line (though i think they mean a different spice haha... ) and i never fill it that much
grind for brief periods at a time as not to burn out the grinder.
if it isnt all powder it should be ok, decent yeilds have been reported using shredded bark.. search shredded bark
try an acid soak before a stb or more and longer boils if your doing an a/b
hope this helps, good luck!
The dude