Crankcase timing tool

Just check the scale in the primary case is accurately set (using a timing disc and a piston stop or dial gauge) Then use that to strobe the engine as normal.
You would have to check the slot in the crank is accurate first (using the said timing disc) anyway as it could be anywhere.     

.. use a similar arrangement for timing. But as Peter said, you need to check with a timing disc and piston stop.

It is not unknown for the ignition timing plate to be out of kilter on some engines.
If you have the means the suggestions above will stand you in the right ball park. Its never wrong to verify previous established practices as often discrepancies are found.
Norton are not immune in this area for inaccuracy. 

(Edited)
I have done just what you suggested and made a 'bung' with a hole for a 3/16 drill.
This then locks the crankshaft at 28 degrees, then you can check the scale in the inspection hole is spot on, as it mounts on slots, it could be out by a few degrees either way..

I think there's a tool available from AN but I centre the slot by eye. The 28° was checked years ago after balancing the crank, it was spot on. Also put a TDC centre punch mark on the crank as a reference.

On my Mk3 I use the crank slot to set the Trispark with the inbuilt LED, a very simple, accurate process. Never used a strobe to check, even prepping for a dyno run.

I marked 28°on the primary as the original scale was miles out. It's more likely this will be inaccurate rather than the factory machined crank timing slot as Peter Finlay suggests. 
A lot of room for error with a timing mark on a 74mm removable rotor made by different manufacturers..

I'm confused by this discussion and wary of questioning others who are much more knowledgeable than I. I've previously asked Ashley (Andover Norton) what position the crank is in when their crank locking tool is inserted.
I was told 30 degrees.
Why was this not covered in the original workshop manual?
  

I would very rarely disagree with Ashley, but there is no doubt it is 28 degrees. Although the crankshaft drawing we have both seen, shows the slot at 30 degrees, because of an earlier question doubting the angle, we have done measurements on at least four 850 MkIII's
top dead centre was very accurately set and then the engine rotated until the peg dropped in the slot, this was exactly 28 degrees.
I have just checked back through the email conversation in 2022 with Ashley, and we came to the conclusion that the drawings are so poor and there were so many changes, that they cannot be relied upon. But I can state that on all 850 MkIII's tested, it is 28 degrees.