Andrew,
That sounds about par. I used one with a 15cm frac and it could generally land an object in a medium power field. I suggest you use >3 alignment stars. I used 5-6 and I believe it improves the model accuracy. Be fussy about centering guide stars. Before I had a cross-hair ep I used a 7mm ortho and out-of-focus stars best estimate the centering. If your finder is well aligned you can use 'precise goto' (or whatever it's called) to hop to the object via a nearby calibration star. Use the finder to put the calibration star on the sensor then centre on the sensor. [I've never tried that but it sounds good
]
David