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How do you equate the sitx bone measurement with actual saddles widths?

I understand you are supposed to add perhaps 15-25mm to your measurement, but is that the actually total width then of the seat?

The reason I ask is that I plan to buy a saddle from some private party -- new or used -- and I mostly see total length and width of the saddle.

For example, I have a sitz bone width of 115mm. If I add a typical 20mm to that to get to 135, do I buy a saddle of that width or something wider?

Thanks.