I hope Ray will chime in with his experience using the blocks. This original post is from some time ago and I know he's spent quite a bit more time with the blocks since then. I have as well, and I can hopefully offer some help with a few of your questions. The block I designed is based on the original blocks produced by Richard Clark and Pete Hidy, with my own minimalist take. But I've found a few things that may help. Or at least confuse the issue. There are a number of waxes that work very well with the dubbing brushes and most folks one the one hand want to know what the correct wax would be, and on the other usually try several and each has some advantages. The image below is the tin taken from Pete Hidy's fly tying kit. You can see he uses a block of straight bees wax, a type of cobblers wax (which has a bit more tack and offers a darker overall effect), and a blend that is similar to the wax I use most often - a blend of beeswax, rosin and a pinch of olive oil to soften the mix. Jim Leisenring used a few different waxes as well, but for the most part he preferred a was with a very high resin content which required some effort to get the bead of wax soft enough. Of course, he didn't use the block, and rolled his brushes on his pant leg, but the waxes silk and twisted bodies are essentially the same principle. The block just offers a cleaner operation. I have a buddy who uses a very soft tacky wax and he does very well with it. Others see little benefit in using anything other than the beeswax, believing the wax is sufficient to help the silk and fibers meld into a cord. They manage to distribute the dubbing and spin the bodies without the tack found in some wax blends. I can appreciate that as well.
On the question of how many flies to get from a brush, typically I think most tiers use one brush for one fly as you try to build the taper into the brush, and with enough practice work it out so the thorax is built in and the whole thing comes together very efficiently once wrapped on the fly. You can even change the dubbing material for the thorax before spinning the body for differnt effect.
You can see the marks next to the background strip. These are marked at half inch increments which helps to guage how long the body would be, or how much dubbing you'll need for any particular fly or fly size. I was spinning bodies that were 2-2.5" long before I incorporated the marks. It's a nice refenence and I usually need all the help I can get. =)
The body above would work well on a size 12, or as I usually do, I just tie in by the small part of the taper, allowing a few turns with no dubbing at all and then just tie until I have the thorax, that way I'm not spinning different bodies for different fly sizes because I use this hare's mask blend for flies 18 - 12. I do have a few flies that I know I will be only tying in 16's for instance, and those spun bodies are usually just over an inch, but that's it. The taper is significant so you really wouldn't use the same brush for more than one fly as you would with a wire brush. (I'm sorry to go on and on, I just happen to enjoy this topic.) There are some amazing wire brushes and you can get a number of flies out of each brush so spinning them the longer the better. The block above can be used with fine wire as well, and can be used to create a tapered body with the wire. It seems a different operation and very different effect than the long wire spun bodies that are common.
This is the hare blend dubbing shown on primrose silk on a partridge and hair flymph.
Thanks for your patience. =)