Hi Ben,
here are a few calculations for your 12", f/4.8 (1463mm focal length) scope:
Max mag(ideal seeing) => 12"*50= 600x with 2.5mm eyepiece.
Max mag (good seeing)=> 12*30= 360x with 4mm eyepiece.
Max mag (usual seeing)=> 12*15= 180x with 8mm eyepiece.
A little discussion is needed here to explain these calculations. The max magnification that is expected from an optically excellent reflector is about 50x magnification for each inch of aperture. This assumes absolutely still seeing conditions(stars do not twinkle at all). Most people can expect their seeing conditions to allow 30x magnification per inch of aperture, as ideal seeing only occurs a couple of times each year. This varies from location to location, thus the more nights you spend at the eyepiece, the better chance you will catch one of these very still nights. Unfortunately, the average night will only yeild seeing conditions that will allow 15x magnification per inch of aperture. This being a fundimental assumption, your 8-9mm eyepiece will probably be the most useful on these average nights of seeing. If you buy a 2x barlow lens, you will be able to turn your 9mm eyepiece into a 4.5mm eyepiece that will allow you to take advantage of the nights that allow 30x mag per inch. A 6mm eyepiece will be useful from time to time, but not often unless you live in an area with exceptional seeing.
The most important thing you can do to ensure you are getting the most from your scope is to collimate the optics frequently(some suggest prior to each observing session). This process is a hassel at first, but becomes easier with practice. Sky & Telescope (June 2002 issue) is supposed to have a clear article on collumation coming out in the next week (may be on the newsstands now). Large, fast scopes are very sensitive to optical alignment related to the steep light cone produced in a short distance. You need to collumate to the highest level possible to achieve crisp planetary views at high magnification. F/8 and higher scopes are more forgiving but still achieving critical collimation is always important if you want the most from your optics. Laser collumators are useful for rough adjustment, but the star test should always be used for fine adjustment of your optics. Everything else is academic. Check out the article in S&T, and here are a few links that may help you out:
Testing optics
http://www.deepskies.com/Index.asp?Screen=Testing
Star Testing Optics with reference to Suiter excellent book on the subject
http://www.skywatchertelescope.com/EducationST.html
Collumation
http://perso.club-internet.fr/legault/collim.html
Collumation 2
http://www.efn.org/~mbartels/tm/collimat.html
Daylight Collumation
http://www.carmelcoast.com/pages/Robin/collim.html
Collumation 3
http://frontpage.dallas.net/~markp/collimation.htmlRichard Snow
