how does Jim Y determine swingweight of a club only knowing raw shaft length/weight, grip and headweight?

for example:

Question regarding STI2 Iron heads: Curious to know what swingweight these heads would be would be paired with Nippon 1050GH *R* flex cut .5 inch long wirh golf pride MCC Plus4 midsize grips? Same question when .25 long?
JimY
ยท 14 days ago
At plus 1/2″ or 38 1/2″ for the 5 iron the swingweight calculation comes out in the C9 to D0 range. At plus 1/4″ or 38 14″ for the 5 iron the swingweight calculation come out in the C8 to C9 range.

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It would be good to know which one he is using. I have a similar system, and I’m off by 2-3 Swingweights compared to Jim’s.