It seems odd that after adding the Riggle adjuster your arm is too high except for thick records. Could you do this:
- Get a lead from an automatic pencil and lay it down on top of the headshell in line with the 3 large holes (assuming you have a Rega arm). It should be basically in line with the cantilever of the cart.
- Get a nice, square business card and draw some accurate parallel lines on the back, length-wise.
- Place the cart down in the middle of the record, with the platter not spinning.
- Put the business card up on edge on the record behind the cart.
- Get down and with one eye closed, line up the pencil lead with the lines on the card.
- Adjust until it is level.
If as you say, if you can't get the pivot low enough then you will have to use the spacer between the cart and headshell. For the Denon 103, some say performance is actually improved on a Rega arm with a brass spacer which adds more effective mass to the tonearm. This assumes your counter-weight can balance it all out without going off the end of the stub.