It's easier than you think. If you remove the cap and clean off the rim along the top of the distributor body, you should find a mark. Literally a little line scribed across the rim. That's your zero point. Get the engine to top dead center, check that both #1 valves are closed (look into the valve cover, lobes should be angled outwards, but up). If you have any doubts about your cam timing, open the top of the timing belt cover and check for the alignment mark there.

Now then: Remember that the drive gear is helical. As you drop the dizzy in, the rotor will rotate. You may have to lift and drop a few times to get the right tooth, but when the adjustment slot in the dizzy base is about centered over the fixing bolt hole, the center of the rotor tip should line up with the mark on top edge of the housing.

DS