As only a rule of thumb for coming up with an estimated starting dose, which should be followed by a blood test about 2 weeks later and then possibly an adjustment of dose depending on results, I figure a good value to be to take the planned dosage of testosterone per week and divide it by 200 mg/week -- e.g. if planning on using 500 mg/week, then the first number is 2.5 -- and then multiply your off-cycle maintenance dose by this.
If the calculated value is over 1.25 mg/day then just start with 1.25 mg/day.
So for example if you are planning on using 750 mg/week of testosterone per week, I would figure (750/200) * 0.25 mg/day = 0.94 mg/day, which can be rounded to 1 mg/day.
If on the other hand you have previous experience, including blood test, with letrozole and a given amount of testosterone use, then go with that experience.
You would not need to taper off of letrozole beforehand.