Gotta put my two cents in
Try winding it up high in second gear or so. That will give you an idea of what's possible. Then, shift at 50% of that, for starters.
What you're trying to do is go high enough in one gear so that when you shift, you aren't lugging the engine in the next gear. It's no more complicated than that.
The good thing is, after a while, you'll be doing this instinctively, so relax and enjoy.
If you ride in the woods, your bike has neither speedometer or tachometer, so one starts with their ear, and soon gets a feel for it.
Happy trails!!! Mac