Primary chain tension

Tension can only vary because of eccentric sprockets because its length is constant at any one time. The up and down movement is very sensitive to the gap when it is nearly tight. A slack chain is better than tight, especially with all that aluminium down there expanding more quickly than the steel chain. The difference between 1/2" and 3/4" is to be expected. It's fine.

As per the instruction book, 1/2" to 3/4" up down movement is ok. As long as it exceeds 1/2" on the tightest spot. Slightly more is acceptable. So Norton knew that some manufacturing differences exists, both on sprockets and definitely on chains. As David said, a warm engine, engine plates and gearbox tightens the primary chain. Caused a number of broken chains on the TT.

I suggest that you look closely at the chains you have. There are several makes that are basically a waste of money and this includes a few famous names from the past, previously manufactured in Japan, Italy and the UK.

The attachment shows my UK manufactured primary chain after less than 300 miles. The rear chain had to be junked after only 1000 miles despite a wax/oil dunk every 3 months and regular spraying.

Go for a good German make.

When you tension (or rather adjust the slackness) of the primary chain, the rear chain should be under load when you do it. I use the subtle method of putting a boot on the lower run. This removes any movement in the gearbox adjustment. If you don't do this, the greater tension of the rear chain when you are running will tend to pull the gearbox back and make your primary chain too tight.

I hope this makes sense...  

... to overtighten the primary first then use the adjuster(s) to push the gearbox forwards till I get the right slack. This ensures that all the play in the adjusters is taken up so the gearbox won't be pulled back.

There is nothing a German officer can't do if he has the instruction book.

Surprisingly, it is all covered in the Norton instruction book. I usually check with it, before answering to a thread.

If length of the side plates differs one thou in manufacturing, up/down movement of the chain can vary more than half an inch between tightest and loosest points. Quality chains recommended.

Renolds allegedly lost interest and pulled out of making motorcycle chains because they primarily make their profit from industrial chains. Modern motorcycles operate at power levels far outside the Standard design parameters (BS, DIN, ASTM and the like) for chains - even more so when they are not even run in a full oil bath. 

I still run Renolds chains. Primary chains as long as there is oil in the chaincase just go on and on. rear chains last up to 6,000 miles (as long as you avoid winter salt), much the same as ever.