Blended winglets

[JackSillitoe]

One to ponder over morning coffee!

Whereas we hear that blended winglets on Boeings improve fuel efficiency by 2% to 5%, by reducing induced drag [Di] (although one can argue that a similar increase in AR would do the same), has anyone come up with a formula to translate Di into thrust/HP and thereby factor the 2% to 5% fuel saving [claimed] into a mathematical solution for varying alphas and air densities?

[XPMorten]

just a note,

Big part of the 5% gain doesnt come from the winglet itself.
The winglet reduces aerodynamic burn with around 2-3% compared to
an acf without winglets at the SAME weight.

However, an acf with winglets can plan less trip fuel than the
one without, so if the two acf fly the same route, the one with
winglets will ALWAYS be lighter than the one without which
makes the total gain 4-5%.

XPM