If you just Google setting front toe-in, you will see there are many ways people have come up with to measure toe. There is nothing special about the M3W when it comes to measuring or setting toe, other than the mudguards getting in the way. Many ideas on the proper value to use, but that's another story. Personally I set mine at 1/8" (+0, -1/32) (or 1/8" toe-in, as opposed to toe-out), and run my front tires (factory spec Avons) at 26-27psi.
As received (used) my machine had fairly massive (3/8") toe-out which made for very darty steering and feathered tires. There is a bit of a myth out there that these machines need toe-out, and I've no idea why that would be so. Moderate toe-out can be useful for track/racing cars to aid turn in, but in a street car you are looking for straight line stability and stability under braking both of which requires toe-in.
With the front mudguards removed you can do a quick (and somewhat crude) check with a tape measure and another person. With the steering straight ahead, and holding the tape as high as possible, and level, under the body work, measure the distance from the center of one tire to center of the other. Move to the front, and this is important, hold the tape at the same height above the ground as before and measure at the front. If the front measurement is less, you have toe-in equal to the difference in measurements. And if the front measurement is greater, you have toe-out.
BTW-while ideally you want to take these measurements at the mid point of the wheel height, for a street machine, taking measurements as high as practical will suffice. Just endeavor to take the front and rear measurements at the same height. s
The device I've been using for some years now to measure toe is this:
https://tinyurl.com/toe-gauge Hmm...I see they are a bit more expensive than they used to be, but what isn't. You can make something similar with surplus materials. In my youth my first homemade version was 2x2" lumber and coat hanger wire. Had to be very careful to not tweak the wires when moving from back to front! I later had a couple of versions of conduit based designs which served for many years before moving on to pro built alloy unit mentioned above.
