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Yes, a focusser with a fine control (usually drives the focusser drawer tube 1/10 of the rate of the normal knob) is very useful. You would need to replace your exisiting focusser. However, I worked without one for a year, no problem. Is your "tightness" screw in sufficiently? The draw tube is not slipping? Another way to adjust focus is to lift the eyepiece slightly out of the focusser. Just loosen the screw and while looking lift it up until you achieve focus, then nip the set screw up. But you shouldn't have to do that. 10mm is not too much. I worked with 6mm in an 8" dob without a problem.
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