Also, not to do too much hand waving, but it also depends very much on the leaf you are using. There is a lot of variability between the wrapper and binder leaves available, and it just takes a little trial and error to get the feel for what works best for each depending on it's thickness, elasticity, etc. The more you roll, the more instinctual this will become. I like to prepare my wrappers/binders the day before I roll. I pull out the number of leaves I expect to use, and put them in separate bags, add moisture with a few mists from a spray bottle, then leave them overnight to absorb the water. I've found this works better (for me) than trying to case them right before rolling particularly if the leaves have thicker secondary veins. It gives them more time to soften up with less water. If they feel too wet, I set them out on the work bench while I build the blank. If still too dry, I give them another mist and let it absorb for 30 minutes. This should be a relaxing activity - don't stress too much about it as you can't really hurt anything. Time will solve any problems.
