Well.. there's 360 total degrees of rotation, so 20 degrees is pretty near TDC. Look closely and the factory may have already marked it for you, perhaps a "T" for timing.. somewhere near the TDC mark on the perimeter of the flywheel. A reference mark of some kind would be on the engine block. If you see one, move the piston to near TCD to find the other.
Scribe as many degree marks onto the wheel as may come in handy someday..