Using the following procedure:-

First thing in the morning before starting the engine, remove dipstick and wipe it clean.

Re-insert fully and then remove to see the oil line on the dipstick.

If you don't wipe it first, the draining oil adds to the apparent level.

At 1ml per mile, that's 1 litre per 1,000 miles which is the lower limit of acceptability, ie when the engine needs serious internal attention. Vauxhall FamilyI 1 engines of that era would do better than 0.5 litre per 10,000 miles when in good condition even at 100,000 miles, at least mine did.