I was thinking to make a suggestion about this yesterday but you've saved me a bit....The method I've adopted is to shine a light (or sunlight in a mirror
), straight down through the inspection hole so you can see the screw bosses,and put in the oil until you an see it somewhere near the boss you need.
A bit off topic now, but similar to check the gearbox oil; By my reckoning, the oil should be around about the top of the layshaft, speedo drive of which can just be seen under the mainshaft.
May as well chuck in the engine oil; Oil level a bit below the return hole (looking through the Inspection hole).
I figured they are called 'Inspection Holes/Covers ' for a reason, so may as well make good use of them