Don't trust anything, get a piston stop and make sure it's right. If you are running a multi-spark ignition, you will need to disable it to check the timing correctly with most advance retard timing lights. My brother bought a boat and trusted the timing marks and detonated both motors.

Agree, first find your diameter and buy a new timing tape - they are cheap and will most likely fall off sooner rather than later, but will give you a good idea
Next you find TDC using the stop method me too ed above and place your tape accordingly

That line is TDC. You won’t need a timing tape if you get an adjustable timing light.

Very true, just about any auto store stocks a digital timing light and they are cheap enough