The easiest way would be to get some well marked brokens and breed them in, but that wouldn't be using what she has.
If the lightly marked ones have a solid parent, then you can breed back to the booted ones and hope the light markings and heavy markings will cancel out somewhat - they'll still throw poorly marked brokens more often than well marked lines though. If the lightly marked ones have two broken (or booted) parents, then they are probably charlies, and wouldn't help you to get better marked brokens.
The last two options you have are to either just take the booteds with the most white and try breeding them together to see what she gets, hoping for some with more white than the parents, or you can breed those booteds or lightly marked ones into unrelated solid lines - one of those solids might be carrying the genes for a proper pattern.
