Blood moon is 7 days, the range is seven days
ahhh this is your problem.
Basicially what this does (as Gamida already explained, although if I am not misinformed it is +- not only +) is that the medium intervall of hordes is 7 days.
But the range makes it so that it could happen 7days before or 7 days after.
Basicially Day 1-14 is fair game. And once there was a horde, it starts over the same (say day 13 was the horde it might happen between day 14 and day 27)
What you want (if you want a predictable horde) to have the range on 0.
This means every 7th day, there will be a horde.
Range is actually a nice feature for experienced players, since it introduces a randomness factor where you can't always plan out your cycle (day 1 finding a poi, day 2 farming, day 3 exploration, day 4 upgrading, ...) since you never know when the next horde will strike.
But for new players, setting it to 0 is prefered.
Have fun ^_^