your test is a good idea, but I heard from a valuable source this:

"The early years 190e has a gauge that indicates a normal pressure of about 2 (depending of alot of factor, like rpm), but relatively to some complain of owner that tought that it wasnt normal that the gauge wasnt at 3, Mercedes changed their gauge so they would indicate on normal condition a value of 3"

I beleive it is right 'cause there is a couple of 190e that has gauge up to 3 in those years on the 190revolution forum.