I have both indicators, Up front - Timing marks on my front balancer, with a pointer.
In the back of the engine, I had my flywheel on a mill with a rotary table, and cut degree marks.
Then, with the engine lined up to TDC from the front mark with the timing pointer, we drilled / tapped a small hole to extend a machine screw to get the rear reference on the bellhousing window accurate.
This way I can check the timing for the back easily.
I also installed a 12 V socket (cigarette lighter plug) of the left-hand side of the engine bay, so that I can give power to a timing light.
I find marks on the bell housing are useful for quick checks, but if you’re going to adjust the timing, you are really going to do it from the front of the engine because you have to turn the distributor and disconnect vacuum hoses.
Rocky