Since it sounds like your climbing is concentrated in the first couple days or so, sou can adjust the pace for the rest of the tour based on how that goes. If you are ahead after the first few days either finish early, take it easier for the remainder, or make an extra stop and do something else. If you are behind, either pick up the pace or cut the trip short at the end.

If at all possible flexible schedules without a set finish date are best IMO. Being a slave to a schedule sucks.

It never occurred to me that it was possible to really calculate what daily mileage was possible or more importantly desirable at least in the mountains. As a result I try to never really plan more than absolutely necessary. That means that I might say, "I either need to stop today at point a, or make it to point b due to the conditions ahead", but never try to plan where I will be more than a couple days ahead.

Truth be told when I do guess what I think I can do on a given day I am often way off in either direction. I have done 142 miles on a day I thought I might do 70 and have done 25 on a day I thought I would do 60.

There are places where lack of water, lack of places to camp and so on will mean that you will have to make a destination. Also in hot climates stopping in a place that allows the hardest climbs to be done in the cool of the morning is a good idea. All of this means some short term planning, but I try hard to only plan as far ahead as absolutely necessary and if possible to have more than one option for the day.

When it is flat as has been said the wind can be a factor and can also throw you way off. If you are not alone drafting can minimize the effect of winds though.

Bottom line for me is to try not to have a set schedule and to take it as it comes, planning only as much as conditions demand. It isn't always possible, but that is the best way to go if you can, at least it is to me.