I wouldn't assume there is an equivalency ratio that holds for all individuals.
I don't know whether there is even a trend, that high or low responders (those needing, respectively, a small or a large dose for a given amount of estrogen reduction) tend to be high or low responders to letrozole.
I would just handle it as a new drug that needs to be adjusted for the individual case.
My method for approximating dosage in a cycle is 0.18 mg/day for each 100 mg/week of testosterone ester, assuming that use is at least 200 mg/week. (If it less than this and this small amount of testosterone is the only steroid being taken, then simply a flat value of 0.36 mg/day.)
So for example if using 700 mg/week, then 1.26 mg/day. But as none of this is super-precise, actually one would use 1.25 mg/day as the starting dose, and check results at the 2 week point. Or reduce immediately if there is reason to think that estrogen is being driven too low.
If desired the starting amount in this case could be 1 mg/day instead of 1.25, if let's say that was more convenient with the particular formulation or if wanting to be a little lower in the guess. Inherently any starting-guess method is not going to be super-precise.
The above is assuming testosterone as the only aromatizing steroid.