Tips for digital SLR camera, how to focus manually to get optimum focus?

You have already discovered that both LCD display and CRT lack the resolution for critical focus. This was a common problem with TV cameras and the solution was to have the operator "rock" the focus. That is they would turn (rock) the focus lens past the point of relative sharp focus until a slight blur was evident, then turn back the other way going through sharp focus again until a slight blur. The critical focus was midway. A little practice and you will find you can do this relatively quickly.

Used lens at 300mm to focus before setting to other lenght like 100,150,200 or 300mm.Don't need to use autofocus.