CH provides a paper protractor with every ignition they sell. Attach it to a piece of light ply that can fit the prop bolt. Make and use a bent wire pointer like CH shows in their ignition instructions. Find TDC any way you want, but doing it with the plug out is easiest. Here's the super easy part. Use the CH tester to visually see whan the ignition fires when you rotate the hub. It's a simple light diode attached to an ignition chip.

You can do all the math stuff you want but seeing when the ignition fires compared to where the piston is in it's stroke is about as easy as it gets. Don't fire the plug if it's not touching, grounded to, the engine. You can put the magnet at any location on a hub as long as the hub can be rotated to set the timing.

Too many engineers spoil the broth and make it appear much more difficult than it needs to be. It's an early part of the basic engineering courses, "making the simple difficult to justify job and security 101".