The eBay FedEx Smartpost divisor is 166 and has been confirmed by Trinton.
The estimating calculator, rates shown in search, listings, and charged to buyers all use the 166 divisor.
I just checked again 3 minutes ago, and the actual label page is still using the 139 divisor (and therefore overcharging sellers for the labels).
There would be no after the fact recalculations by FedEx - as far as they are concerned, all eBay FedEx labels are rated using the negotiated 166 divisor.
It's going on 2 months now with no fix in sight.
The only way to make Smartpost work without losing money (or grossly fudging with a handling fee) is to use "fake weight" on listings where Smartpost is the only shipping option.
https://community.ebay.com/t5/Selling/WHAT-I-DISCOVERED-TODAY-REGARDING-CALCULATED-SHIPPING-VS-ACTUA...