Linkage spring rate accuracy coil vrs air

Hi Folks,
I am working on building a single pivot mtb as my 2nd full sus build. I use linkage to work out the design, I am just wondering how accurate people find the sag/spring calculator?
My first frame was a link actuated shock and to be honest I got two of the pivots slightly in the wrong place, I therefore needed a heavier spring. Lessons learned, so a single pivot this time to help reduce the weight mainly but also the chance of me cocking up. I have it to a 500lb spring for 20% sag but when I convert that to air shock it makes no sense to me as the pressure is huge!
Anyhow, just wondered about others experiences with this.
Thanks all.
Tally