Dommie 99 Oil tank capacity

is not enough

I looked in Roy Bacon 'Norton singles', which lists the oil tank capacity of 1958-63 ES2 and Model 50 (the featherbed ones) as 4.5 pints.  Your 99 will have the same tank. 

I would suggest filling the tank to 3/4 full, prime the feed oil line by loosening the feed at the engine until it leaks a bit.  Make sure you toss some oil at the cam followers. It's a good idea to use a pressure can to fill the rocker oil feed as well. If you have had the motor apart, I hope you filled the crank.  If necessary you can take the timing cover off and squirt oil directly into the end of the crank to remedy that. If you do this, make sure the pistons are at BDC.  It may be possible to prime the pressure feed without removing the timing cover, there's a bolt below the pressure relief valve that fed an oil pressure gauge on the earliest bikes. I would suggest taking the spark plugs out and kick the motor a good few times to prime the oiling system and make sure it has pressure before you try starting the bike.  It's a good idea to put the plugs into the caps,and rest the plugs on the engine when you do this, so the HT circuit is complete, or you can damage the magneto or coil.

Once you have started the bike, there will be oil in the engine, and the level in the oil tank will have gone down.  Remove the oil filler cap and check that oil is spurting into the tank.  Put your finger on that outlet pipe and push the first return oil up to the valve gear.  Take a look at the oil level, and note how much it has gone down.  That oil is now in your engine. Don't run the bike with the the oil level below half-full.  If you are going on a long run, raise it to 3/4 full.  Take some extra oil....

If you leave your engine for a month without starting it, all the oil will be in the engine.  Do not forget to check that oil level before you start the engine, because if there's none in the tank, there will be no oil pressure in your big ends.  Big no-no. 

Until you really get to know your bike, you will not know how much oil it uses on a run.  You may be shocked just now much the oil level drops on a long run on a hot day.  Check the level when you refuel.  One time while refueling, I found my oil tank was 1/4 full, having started the day well above 3/4 full, 300 or so miles back, with another 100 to go. Fortunately I had a litre bottle of 20W50 in a bag

Much depends  on the temperature and your throttle hand. Normally my 99 would go between oil changes without a top-up (1,500 miles). However, in my student days I once rode from Bristol city centre to Edinburgh city centre in 5 1/2 hrs with minimal use of Motorways. This involved long stretches of full throttle. At the half way point I needed 2 pints of oil to top up the tank. And the same on arrival. Wouldn't do that now... 

That looks really lovelly , must have been one of the first alternator /coil bikes.  

A beautiful rebuild. Take the early miles gently until you know it has bedded down properly and keep changing the oil. I have inserted a tap/ignition cut off switch from Kingpin components to prevent oil draining back into the crank-case, and that is also good for peace of mind. Consumption is definitely related to throttle opening!