Tire widths on HED Belgium plus 650B??

[RipVan Rando]

Hi, I am new to this group and am doing my first conversion to a 650B from 700C and am about to buy some HED 650B rims to lace up but I know from experience with these rims that tires tend to get a little bit plump or puffy.  I could use some help.....

I am wondering if anyone has actual width and height measurements of 38 or 42 mm nominal tires on these rims?  Compass or Soma EL would be nice to know.  I am certain 38 mm will work but would prefer to go all in with 42 mm tires if possible.

I know I could use other rims but the HED Belgium plus is my first choice, then A23 OC, and then maybe Grand Bois.  

Thank you in advance for any measurements or even hearsay would be nice.  (I did search)

[RipVan Rando]

Soma Green SL measure 41 mm at 45 psi with butyl tubes