DOC capability depends on the material and cutters involved. And the required finish for that cut. For example, a roughing cut that you are going to cut again anyway, can look like crap and be fine. While if you want a nice finish, you might need to shave a thou.
For roughing cuts, I take whatever doesn't stall the motor or cause too much heat. In 12L14, that might be .250 off the diameter with a sharp HSS tool. Now, I don't generally do that, there's usually no need. A couple extra passes doesn't hurt anything, I'm not in a huge rush, so it's rare I run more than .100 even on soft material. At the same time, I have some really hard steels here that I can't do more than about .050 on, and that is pushing it. Large DOC can be hard on tooling, and the machine, so start small and work up based on how things go. Particularly for less familiar material.
Even if you are not planning to use HSS, you might skim over some of the discussions in the HSS grinding thread in the beginner section. It covers a lot of info on tool pressure and how to limit it. That translates directly to how much force is required to cut. Minimizing the forces as much as you can gives you more capability, all other things being equal.
I don't have the same lathe, mine is a PM 1127, so quite similar in rigidity I would think. I also replaced the compound with a solid block, so no flex there.
