OK, been away messing with a 'new' (to me) '75 Opel. Yes, you're finding that that the play in the lifter plunger is getting in the way of accurate .050" lobe lift measurements. It will be the same issue at the retainer.... so going there does not fix this. And I would not count on the 'squishiness' being equal on both sides of the center line of the lobe. If you can take the plunger out of one of the lifters, then go to the hardware store or root round in your junk pil and get a long enough piece of rod or tube and figure out a way to rigidly put it into the hole of the lifter body so it will reach far enough for your dial indicator, then that is what I would do. (But I have never had one of those lifter to know how easy or hard it is to take out the lifter's plunger.) If this measurement is going to be at all reliable, it needs to be accurate; it takes some care to get accurate readings on the lobe centerline.