There's no real set rule for it, but I'll try my best. Basically, it has to do with how close to the subject the viewer percieves themselves to be. So, the closer you get, the more actual space of the render is going to be covered. You'll want to have a setting that makes the texture map in use at least as close to the viewer as the figure the map is applied to is. hmm that's worse. um, if the face fills the whole screen, use bigger numbers to pull out more detail, lol. Good rule of thumb. It's one of those things you haave to experiment with to get the best results from for your particular image. That said, after asking the other memebers of Odd Ditty Foundry, We'll toss out another rule of thumb (this one from ynsaen) "set the MTR value to equal the size of the largest texture map in use in the scene, and you'll be fine". so, if the largest texture map in use in the scene is 1510 set your MTR to 1510. If you go higher, you will sometimes get blurry effects (just like you would if you enlarged the map), while if you go too low all the details of the texture will be lost.