The fill level is not all that critical. If you overfill it, it'll just suck out the extra and burn it. It just has to have enough to inhibit the upward travel of the air piston. To see what they're talking about in the instructions, remove the air cleaner and the piston damper plunger rod. (unscrew the plastic or brass bit at the top and withdraw the plunger) Now, look down into the place where you just removed the plunger and stick your finger into the carburetter's mouth. push the air piston (the cylindrical thing blocking the way into the manifold) up. What you see moving upward is the hollow rod which should be filled to within 1/4 inch of its top.
A quick check of the level can be performed without looking at anything. Just unscrew the damper rod and lift it about 1/2 of an inch. Now, with it nice and straight, push it back down. If you feel resistance you have enough oil for it to be functioning correctly. Resisting the upward movement of the air piston is, after all, the job of the damper piston along with the oil. We're just pushing one thing down instead of pushing the other thing up. Feeling resistance before the thing is even seated shows you have some reserve. SUs are great carburetters. They tend to be just a bit under-understood. Should I do a whole thread about the operating principles and how to live with them?
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon