Practise!!
Another idea is to use a Ronchi grating on a flip mirror. Set the CCD to your "best" Hartmann focus and then put a Ronchi grating on the guide port of the flip mirror, adjust until you get the gray null image of a star and lock in place. If the focus is spot on, all you need to do the next time is set a bright star to null with the Ronchi, flip to the CCD and away you go!
If you find the focus is still "soft" adjust it slightly and note the view in the Ronchi ie three wide bands etc. When it's 100% then re-adjust the Ronchi screen ( without moving the focusser) back to a null. Then you have a great reference for the future....
With my Gstar I use "point and zoom" to achieve focus.
Put the mouse on a star to magnify it, and then adjust focus.
I get live view from my camera to the puter screen, I don't know if you can do that with your camera though.
There's a couple of different versions for free download here http://www.hsinlin.com/software/mouse.html
I find I get good final focus by picking a star that is just on the verge of being too faint to see on the screen and getting it to come in and out of view. When in focus the star appears and when out , it simply dissapears. Its quite accurate.