From what I've read, again not experienced, because I work with metal and solution at a constant near 70deg.F, with an increase of 10deg. you can as much as double the effect of the reaction. This in mind, I can understand Fred's need to dilute solution if working in a garage in Florida.
The only changes I've required seem to be related to the make up of the actual metals involved. So far, it's been a difference that is fairly subtle and can be seen after the first immersion and wet carding. I've installed a flourescent light directly over the sink and etchant tank set up. This allows a good view of whether the result is little color left, decent contrast starting to show, etc. From this, I've been able to adjust bath time or rusting solution strength slightly to get better results on the next application. This can cost the time involved in an application or two, if the first result is too little color, but so far, that's about it. I have had one case where you could only push the metals involved to a certain point, or things started getting very blotchy and uneven. On those barrels, I had to remove the finish, repolish, and start over. I then cut back rust time and rusting agent strength, went through more applications to get all the color I could, and stopped the process as soon as subtle signs told me I'd pushed them to the limit. That set taught me that lighting and close attention to what the barrels are telling you is crucial. I might also add, here, that thoroughness in the wet carding is also critical. Anything not done well will show up more strongly on the next application.
Concerning logwood, I've found no need to use it on the black finish. I've tried, and found that, in my opinion, the benefits of gaining color a little more quickly are outweighed by it's potential to take away some contrast. I may try it again, but so far I've just gotten what I needed through more applications.
I've used logwood, in the past, on simple browning, to decent effect, but haven't attempted it yet on brown damascus. The main reason is that I'm looking for an agent that produces a lighter color rust than the one I currently use, first. My Zischang solution tends to render a chocolate brown, and I'm afraid the logwood would turn that to black. When I find the right stuff, I'll shoot for a plum color with the logwood.
When working on the brown finish, Gaddy, Angier, or both, mentioned that at times one would need to rust and card two or three times between etchings. I haven't done many damascus browns, but so far have found this to hold true. Basing the need to etch on the amount of color you retain after carding seems to work. Yes, I'm sure better judgement on this will come with experience.
I hope this answered your questions. I'd like to add that my thoughts on this process seem to run long, and in the end give no black and white answers. This, I believe, is the nature of the process. I might also add that the length of my responses should in no way suggest that I'm an expert on the subject. I'm simply relating what I've managed to learn so far. I'm confident in getting the results I'm looking for, but fully realize that my procedures may change as I run across more potential problems.