A very important point Hiko. It certainly makes a big difference. They roll much easier with a side load.

Just use the end of your callipers to measure through the bearings and up against a flat surface.
Subtract the width of the two bearings and this gives you a starting point.

A bit of final tweaking/shortening, is sometimes needed to get them to spin with very little friction. I use a rear LLM axle in the vice and a standard 20mm nut to set them up.
I can be time consuming on some wheels to get them to not have any side load on the bearings. Each wheel is a bit different.

I use stainless steel curtain rod/hand rail which is the old 1"OD and 1/16" wall for mine, as that's what I could find laying around.

No Fallshaws are like that also

I bought 2"bearings R16-2RS from Ebay for AUD$60 (for 8- it was cheaper that way). A little bit of light oil and a block of wood and they tapped straight into the Fallshaw rim. I also bought some reducing bushes 1"to 20 mm from my local Fallshaw supplier - job done.

I still need to make the spacer to go between the bearings. I have some 1"aluminium tubing that will do the trick.

Hey Chook. You sure know your bearings. Yes, they are only 1/2"wide.

However, my recesses appear to be only 1/2"deep (13 mm). I note from an earlier post from Tryhard that he measured 15 mm. I can't explain the difference.

The bearings are seated squarely on the inside lip and are flush on the outside.