Yeah, more white = lighter color = harder, more sandable final result. I was probably using it at about the 60/40 white/black ratio and was pleased with its sandability, able to get a perfectly smooth transition from metal to putty as two sides of a smooth plane. These aren\'t the best pictures, as they were intended to illustrate other things than this, but here\'s where I was using that ratio to fill out lower torsos, rebuild thigh pads and elbows, etc.:
And after sanding and smoothing then primering, the metal-to-putty transition was invisible (to the touch, as well as sight):
The primered figures might look a tad rough, but that\'s because I\'d already played 5 games with the primered team, so it just got worn in spots before the primered pics were taken -- as well as the low-res photo quality of the pics themselves. You should be able to compare the middle figure in the top pic to its primered version (labeled 6\'3\"), though, on the torso and front-facing knee pad and see the transition (or lack thereof).
As for the working times on the chart Einon linked -- I agree something\'s doesn\'t look right there. But as I was only working with it so far on these types of smaller conversions, as opposed to full sculpts, I don\'t recall working with any mixed portion for more than an hour at a time, so it was never an issue for me.