[Mgs] Calibrating the MGA tach

[W. David Houser]

Listers,
A fellow MGA owner has posted this query.
I haven't a clue but want to impress him with my vast knowledge.  
That's why I post to the list:

"I want to know what the ratio is from the camshaft to the tach.  
Actually, how many revs of the flex shaft correspond to, say, 100 RPM.  
If I can establish this, I may be able to confirm, with a speed  
controlled motor, whether the tach is accurate. If it is, then I can  
make some marks on it, corresponding to major speed points."

How's that for an obscure number?
Any one able to answer this for him?
Cheers,
Dave Houser