I try not to use more than half the total volume of filler any more and it gets way too thick.
I mix the filler into the B side first ,making note of the volume before and after mixing the filler. then add the apropriate ammnt of A side. Microspheres are non absorbant so mixing them into either component will not disturb the ratios, it will just throw off the measured volume. Trying to mix the filler into the A&B after they have been combined is no good as you will be wasting time folding the filler in and trying to beat the clock before the resin kicks.
M