You're asking about setting duty cycle for the Bosch KE Jetronic. The basic rules apply to most early 90s Mercs, 24 valve included.

I'm busy right now so can't respond at length. I've measured 50% like you, followed by 50% +/- 3% when idling. The car runs fine but a year ago I saw the 75% that you mention (engine off). Don't know what's changed.

Keep searching as this has been covered at length elsewhere for 190s, W126s, W124s etc. etc.

Bon courage.

RayH