I'm playing around with the following method. I like this approach because the VTA adjustment can be made when the stylus is not on the LP.
Raise or lower the arm until the top of the arm is parallel (using a home-made ruler device, or the acrylic device Brf mentioned).
Get a Stanley laser level (
https://www.homedepot.com/p/Stanley-Man ... /205764000). They're available at Lowe's, too.
Place the laser level on the LP, on its side so that the laser makes a horizontal line. The little plastic protrusion on the side of the body has to be shaved off with a hobby knife.
Make the laser shine on a part of the arm that goes up and down as the VTA is changed. With my cartridge, the laser catches the arm rest hook when it is rotated to the max "up" position. That's your reference point. Mark the line position on the hook (or take a photo of the laser shining on the hook).
(If the laser does not shine onto any part of the arm that goes up and down, one could attach some kind of a ruler or scale to the arm.)
When you change LP, put the laser on the new LP. Since it is a different thickness, the laser will light up a different part of the hook. Move the VTA up or down until it hits the reference point, like before. Refer to the photo, if needed.
That's it. You've moved the VTA up/down, to account for the LP's thickness. Make final adjustments by ear for each LP, to account for different groove depths, and angle at which the LP may have been cut.
(The laser beam will be quite thick. You can reduce the width by making it pass through a narrow gap. I put two strips of electrical tape on the front slit, leaving a gap of about 1 mm. I'd love to find a laser that produces a really, really narrow beam, or focused a spot.)
Memo to self:
1. Make some optical thingy that will amplify height differences. May be a prism...?
2. Make the laser shine onto some sort of light detector, and have some "brains" that will move my VTA adjuster automatically.