Check out this post on electronic VTF scale. These are more accurate and easier to use.
I tested mine with a thin steel strip, known to be magnetic, balancing it like a tonearm over the scale surface. I couldn't detect any magnetic attraction at all. The scale is basically a plastic platform sitting on a strain gauge - there's not much to it. For $20 and reading to .1 gram, it's amazing.
My cartridge is MM but yes, it goes right on the scale. On my Music Hall I have to remove the platter to make room (the scale is about .5 inch thick) but that's a small sacrifice. If it's important you could put something under the scale to bring the surface up to the exact level your platter would be. I thought the difference to be inconsequential.