Here's how I came up with that:Given that every 1mm is about 0.0394 inches and given the thickness of the steel backing plate on my E39 was 0.1005 inches, the minimum measured thickness of the backing plate + friction material calculates to 0.0394 + 0.1005 which comes to just about 0.1400 inches.

If new brake shoe friction material is 0.137" thick, then with a 0.1005" backing plate, the nominal new thickness is 0.2375 which I'll round up to 0.240 inches.

Note: Since my brake shoes were 0.2375 inches thick (pad + steel), which is no where near 140 thousandths of an inch, I was right not to replace the brake shoes at the time I measured them. Interestingly, it seems my brake shoes, at 238 thousandths of an inch must have been almost new (based on cn90's statement that new material is about 0.137" thick.
This picture is from the attached E36 brake job DIY which included parking brake shoes.