Mike has the right idea, the right angle adapter or the flexible wand handle allows you to get truly parallel with the track and get a good perpendicular cut.
I go through a lot of cutoff wheels, so I buy them in quantity here: Reinforced 1.5 inch Cutoff Wheel - 1/16 inch holes - 100pc. While not "quite" as consistent as the real Dremel brand, 100 for $12.97 is a lot easier to live with. These work just fine for me. I also have them with the 1/8" hole for larger mandrels from the same source.