Solo combs are not very good for thinning. Best to get a mane and tail thinner/rake and just comb it through from underneath to thin it. Got mine from www.clippersharp.com
Either spend ages doing a few hairs at a time, but to be honest, I now do people's horses manes for them with the clippersharp tail thinner and find it a better tool.
Dont even try... get yourself on to the clippersharp website... order a rake.. they post it out by return... and you will wonder why you ever bought a solo comb!!!