Hello NH,
a marked change or fluctuation in BP would be in the resting state, as this is when the most accurate readings are likely; so first thing on arising in the morning would be preferable.
So from one morning to the next, if there is a sudden change (up or down) and mostly downwards, is a strong indication of the need to terminate the fast.
Any changes that are gradual over a period of a few days/weeks should be of no concern, but where BP usually lowers itself significantly over the course of the fast, this is nothing to worry about, unless of a sudden nature/drop.