What stance angles are you using? 21.5" seems about right for someone your height.
That said, as long as you've got small(ish) feet and don't need to slide your bindings toward either heel or toe edge, turn the disks 90 degrees and use them for micro adjustments of stance width.
Sliding them each in a notch should bring you to about a 21" stance and keep the setback as per reference spec.
If you do need to adjust heel to toe edge, I'd say go for the larger setback since you said you do very little switch riding.