If you do a drain and refill you are doing about 1/3 of the fluid each time (well, more, but then it mixes with the old stuff). That is a pretty safe amount, and as you mention you are not even to ford scheduled maintenance for normal conditions (not that I agree with it, 150k is a long time!).Thanks for the recommendation! I am kind of leery about taking it to the dealer about hearing about one put V in it instead of LV. May do it myself!!
If you want to do a full fluid change with draining it, you would need to do 3 separate drain and fills. It isn't a bad choice to do 1 drain and refills, Ford shows the 3 drain and refills for issues where you want to end with mostly new fluid.
With the 6F35 transmissions I do a single drain and refill every 60-80k miles, but this next time I will probably do it twice, maybe that third time. My vehicles don't have a dipstick and it is a bit harder to check the level after doing it that I hate doing it.