It sounds like your playing with VTA and that should not be set by the arm manufactures guide lines you should set it to the specific cart your using ( spacer if needed) the height of the back of the arm raising or lowering will affect the stylus angle in the grove you want this to be approx. 3 degrees so set the height based on the stylus angle, this can be also set by listening I'd google VTA set up and adjust the rear height as needed.
Also keep in mind adding a spacer will add weight to the effective mass and will also effect resonance frequency of the arm cart system, in your case this could be helpful.
Looking at
https://www.vinylengine.com/cartridge_resonance_evaluator_results.php?m=Shelter+501&cm=8.1&d...I'm thinking your really close to being in the bad resonance frequency range putting that cart on the SME 309, google shows 9.5 grams effective mass that's not in the happy range for resonance.
I hope I've made a mistake here on my search for numbers and your ok but please double check.
Glen