200 kg for 765 LT at 150 mph

This is almost correct, I believe, based on McLaren's own percentage-based numbers, though they have never quoted a 150 mph downforce number as far as I know. Following McLaren's downforce percentage increases (at 300 KPH / 186 MPH) and using two significant digits for rounding...

MP4-12C was rated at 120 kg

650S - 24% more than 12C = 150 kg

720S - 50% more than 650S = 220 kg

765LT - 25% more than 720S = 280 kg

BONUS: 675LT - 40% more than 650S = 200 kg

So if we scale off of the speed using the fact that downforce is proportional to the square of velocity, 150 mph would yield a downforce of ~180 kg based on the 280 kg @ 186 mph figure.

As I said, close.

For anyone interested in calculating the approximate downforce at a given speed (at least to first order), use the following formula:

Est. DF = Ref. DF * (Est. Speed / Ref. Speed)^2

Est. DF = estimated downforce

Est. speed = speed at which you want to know the downforce

Ref. DF = known downforce at a known speed

Ref. speed = speed at which you know the downforce