Heres the extreme basics. Tune the engine as best you think it is. Then check to see if it's any good. Let the engine idle. While its at an idle either remove the fuel line from the carb or pinch it until the engine quits running. As the engine quits listen to the rpms. If the rpms increase as the engine quits,low end is to rich. If the rpms just go down and quit, low end too lean. I set mine for a very slight increase.

Now each time you do this, re adjust the high end to bring it all together, your engine should be tuned pefectly with this