DIY Truing, Dishing Stand Experiment
Yes, this is a reasonably accurate way of doing it. Along similar lines, with the axle supports square with the base and everything reasonably well aligned, you can just use a vernier caliper to measure the distance between the support itself and the rim sidewall.

Dish sticks have the upside that they produce a very accurate, fast reading with a simple tool, and because of their nature, the precise dimensions/alignment of the tool itself isn't what makes them accurate. Getting the exact number isn't really what's important; getting a reading that you can trust is reflective of reality is, and dish sticks are good at that. I've found that when doing the same thing the "math" way, either with a ruler or a caliper, it winds up taking longer and being more fidgety. But it's totally fine for home/occasional use.

Dedicated dishing tools also have the nice feature that, when they're flipped to the "gappy" side of the wheel being dished and you touch one end of the tool against the rim, the other side is showing you a picture of how much more you have to bring it over magnified about 4x (2x because you're touching the tool to one side of the rim, and 2x because to correct a dishing error of the rim surface being Y distance away from where it's supposed to be, you need to bring the rim Y/2 over to one side.) I think that probably makes it a little easier to get it dead perfect. In other words, they make it easy to see what are actually very small errors in dish, which can be a little hard to pick up on with a ruler or caliper. How much errors this small matter is a separate question of course.