Check this out, it might help you. I also dont have a tach yet and I just tuned my 372 after doing a muffler mod. In the video you can see and hear me testing for the proper sound of a good rpm range. This is done by cutting and then quickly lifting the saw slightly so as to "unweight" it or not put any load on it. It should 4 stroke while being unweighted, and that is the sign of a good tune. The 4 stroking is very important in telling you if you are too lean or too rich. The saw should 4 stroke at full throttle with no load as well. If it's a little boggy, lean it up by an 1/8th turn or so, and if it struggles to 4 stroke, richen it up quick! The tune in the video is where I decided that the saw was at a healthy rpm, so I havent made any changes since. I hope this helps, and here's the video
As a side note, Brad Snelling suggested to me that I run my fuel mix around the mid 30's:1, because the bearings require a little more oil. I run 33:1 for my 372. A little more oil never hurts!