Tren is pretty bad ass AND pretty toxic. Make sure you do your research and know what you're sticking, what it does and what it's side effects are. If you do not have experience with the compound, run the short ester (Tren Acetate) and run it at a relatively low dose. As for a stack it depends on what your specs are and what your goals are. With anything, you want/need a test base. The only "best dosage" is the well informed, tolerable dosage. My numbers are going to be vastly different than your numbers (I normally go 2:1, test:tren) and then next guy's and so on... Once you get some tren in your system, you can adjust depending on how you tolerate it. When starting out, I recommend a short ester so that if you do not tolerate it well, you can bail. As for a rule of thumb, the only hard and fast advice is research and caution. Knowledge is power and slow and steady will get you there.