I had manual trim on my RV6 and here's how I did it.
Set the trim tab in trail.
Make a mark on the metal slide of the trim vernier right where it goes into the panel mount. (use a file to make a permanent mark.)
Then, take the vernier end apart, which will include the slide piece you just marked. This is easy to do and only takes a few minutes.
Then, chuck this piece into a drill press or lathe and hold a file against the mark. This will make a nice groove all around the slide. You don't need to make it very deep, but you need a groove that you can see.
Once this is done, you can pint the inside of the groove red or white, I chose red.
Reassemble the vernier cable and use the mark to always position the trim tab in trail. You could also put the mark in takeoff position, if it is different than in trail, or use two marks of different color.
You can then placard this any way you want, but I never did.