Willster: Even if it weren't for the magnetically induced error of the Shure gauge, it doesn't provide the kind of accuracy I think you will want with your arm/cartridge if you want to achieve the best results. I have two recommendations:
There are many lower-priced digital guages, some here on Agon, sometimes at auction. I bought an Audio One Digital Stylus Pressure Gauge here at auction from "audioparts" for $157 about 3 months ago. It measures out to .01 gram (1/100) which is what you need for a MC cartridge. It's nothing fancy, but it does the job -- and for less than a tenth the price of my cartridge.
My other suggestion, and I wish I could be more specific, is that there are a few simple balance scales out there(like the Shure) made of non-ferrous materials. You'll have to do some Googling.
Make no mistake, the magnets in MC cartridges are extremely powerful for their size. The minute you revealed you were using a Shure gauge, I knew what the problem was. I've been there!