Start by deciding what freewheel you're going to use. Check and/ respace the hub so the distance from the freewheel shoulder to the locknut face is the same as the freewheel width +3mm clearance. Or lightly spin the freewheel on and fit the hub into your frame and make sure a chain wrapped over the outermost sprocket clears the frame with about 1mm clearance.
That's the spacing you need or want, and you can now measure the hub the usual way. Don't forget to confirm that the OLD matches your frame.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.