how to get the SME 309 tail down-help

Have mounted a shelter 501 on my just- acquired SME 309.The word is set the arm tail down for this cartridge but I can't find a way to do that. I start with the tonearm level while the stylus is on the record-as per the SME instructions- using their template At this point the rear bottom  of the tonearm -, which is tapered downwards towards the rear- is about 5mm from the record surface ,which is where SME recommends it to be. If I try to lower the arm from this point I have only 1-2 mm max  before I am in danger of the arm touching the record as it traverses it during play. If I had a warped record  or thicker record it would likely touch it. I tried putting a mat under the record to raise the catridge end but of course this lowers the pivot end of the arm to a position too close to the record surface, as above.Help please??