HiLeft-
We have customers who finger shoot the Max 3000 39.75" ATA, and have no problems with string angle or finger pinch. Yes, very hard to find bows much over 40" nowadays. Even the Supernova is only 40.25" now with Omega cam.
To correctly measure your drawlength for a Merlin, (who measures by correct AMO standards for drawlength) do this--
Take the bow you have now, and run a straight line up from the deepest part of the grip to the shelf near the arrowrest, and mark that spot with pencil or tape or whatever. Next, take a long arrow, and using the same equipment (or fingers, LOL) that you would use with you new Merlin, draw and anchor comfortably. Have friend/wife/whomever mark the arrow in the same spot as the marked spot on the shelf. Let down, and measure the arrow from the inside bottom of the nock where it touches the string, to the mark on the shaft. ADD one inch and three quarters (1 3/4" VERY IMPORTANT YOU ADD THIS!) This is your true AMO drawlength. Check it a few times to ensure you are correct. If you do this procedure correctly, when you order a Merlin it will come through exactly the same at correct AMO standard measurement.
You are correct, no-one measured the same, and not much has changed even today-- there are no standards except for AMO, that's why Merlin uses them. It's pretty simple to figure once you've done it a few times.
Hope this helps. Good shooting, Pinwheel 12