Pryso, Dennesen made another device to accurately locate the arm mounting hole. It was like a 2 sided ruler with a long slot in the middle. One end tightened over the spindle while staying at zero. The other end had a clamp where you could insert a pencil or stylus to subscribe an arc on the armboard. The clamp could be tightened anywhere along the slot to accurately locate mounting hole.
I believe the SoundTractor was the first device of its kind, and the prototype for the ones like it - pivot pointers. A GeoDisk works on the same principle but with a different kind of pointer. With some of these newer ones you have more options for alignments and they might be easier to see.
There is an inexpensive magnifier you can pick up. It's called a linen magnifier. They come in various powers and stand at an angle - could be helpful. Get low power though, like 5X ? Stronger lens is hairier, has a shorter focal length and is more liable to optical/user error. I use reading glasses of various strengths. Magnifiers and uneven lighting can mess up a careful alignment and I always double check with a conventional protractor. Depending on your arm sometimes the pointer goes off center pivot, you misalign, and the pointer goes back and looks okay when you put it back in the arm rest. 30 years ago a metal SoundTractor was $100 and the tonearm hole locator was $200.To answer your question, no reason to upgrade with your situation.
An arc protractor is predicated on the proposition that your arm is perfectly mounted - factory. Otherwise it's not only useless, you'll wind up misaligned. Not recommended for DIY tonearm mounts.
Regards,