... I have had the same problem, with my old blue scope
figuring out what magnitude a star is isn't easy. so get yourself a KICK A55 star map. one that shows only upto a cirtain mag. to do this i figure to look at a known star... yes i can see it , find the star, look at its rated mag. theres one point. then do this untill you cant see a star... (look for patterns that can guide you in). so once you have found your limiting magnitude in your area then you can look at star maps and go I should be able to see that, look for the group of stars pick it out and then "Hop" to the next. there was a set of star maps on IIS forums from a japanese dude. they are good when printed out on A3 and laminated.! Hope that helped
Brendan