Here's a guide - with the eyepiece view, the asteroid will be similar in brightness to the faintest stars shown. You've probably got an inverted view but I've included the corrected view in case you use an erecting prism.
The red circles are the approximate field of view using the 28mm eyepiece but not exactly. It's similar magnification to my scope with a 40mm eyepiece so I used that field size.
Hope that helps, and that you can see enough stars in the sky to work out the rough position of the asteroid in order to aim your telescope in the first place! Good luck.
Cheers -