I'm in the middle of a three-day free promotion. Next time, I think I will do it only for one day and see what happens.
I think signing up for Kindle Unlimited and going exclusive is already helping somewhat though. Only time will tell for sure though.
I'm not going to give exact numbers, but even though my book was only on Kindle Unlimited for a day before I did the free promotion, my sales numbers and my Kindle Unlimited numbers have both already gone up compared to what they were before I did all this.
And the free promotion is going good so far, too. :) I don't know how much it will boost my rank, but it's already going better than I expected it to.