I've got a 130mm diameter newtonian reflecting scope. According to the internet, the maxium useful magnification of this should be 260x. (I'm aware that I may not get close to that depending on other issues and influences.)
Anyway I have 10mm, 12.5mm, and 25 mm eyepieces, and a 2X barlow lens. I did some working out and thought that if I get a 5X barlow lens I could get close to maximum magnification.