Be careful using translucent domes if they're out in the sun a lot; I lost a bunch of cuttings to cooking early on by using clear domes at first, not in a shady place. Now I only use opaque domes where there's regular amounts of sun. All my domes are vented, which seems to work very well.
I settled on a sandy loam for my potting mix, basically a 2:4 parts ratio of sand to bagged potting soil, because it both retained moisture and drained well for me, while being readily available. Another bonus is that the soil falls apart easily when I go to separate the rooted cuttings (unless the roots are very fibrous), which is very helpful.
This is the first year I have done anything with bottom heat, so I can't tell you of it's value by experience. But I have read that Rhodies were hard to propagate until the Nearing frame gave them both the shade and cool tops/warm bottoms that they needed to do well. Considering that my summer propagated azaleas haven't been difficult at all, but the evergreens I've tried have been very slow to root, I suspect that bottom heat may very well make the difference that I'm looking for. I'm guessing you'll get a lot better success by trying it.
I use dip-n-grow on any of the potentially harder-to-root items I propagate, of which Rhodies are included. It is a liquid solution that I have mixed up in a spray bottle (at the semi-hardwood or hardwood rate), so it stays fresh for every time I use it, and I can avoid waste (except for overspray). The only difficulty with that is that it gums up the bottle's spray mechanism easily, so I have to rinse it out well with plain water after every time I use it.
It's pretty neat to see how well so many types of landscape cuttings can grow into plants, when they're given the right kind of environment to do it in! That's what I've been discovering, and it's kind of amazing.
It's fun to experiment with everything.
You're welcome!
