I did the same type of calculation to pick my FP+ times for my trip later this month and came to the same conclusion.
In September my strategy might be different - I will redo the numbers when that trip gets closer - but for a trip on a crowded day I found that it is better to use my FP+ later in the day, even if it costs me more FP+.
For me, it's not about time saved. It's about time spent. I'm never going to wait in a 120-minute SB line, no matter what time of day it is. I look at how much time I spend in line. I am pretty much not waiting more than 20 minutes (possibly 30) for anything. So, if extra FPs allow me to do a larger number of attractions with a 10-15 minute wait, it is advantageous to me.
So, my numbers might look like this:
EP:
Ride TT SB in SR at 9:15--20 minutes spent
FP+ for Soarin at 10:00--20 minutes spent
FP+ for Mission Space 11:00--15 minutes spent
FP+ for Turtle Talk at 12:00--20 minutes spent
Mix in SB rides on LwtL, SE, etc if waits are low
Hop to MK:
Grab FP+ for JC or HM
eat lunch
ride HM. Get another FP+
keep repeating as possible, with rides with no SB waits (like People Mover and train) mixed in
If I save my FP+s to use from 2-5, I will lose the chance to FP+ 5-6 rides, compared to using them from 9:00 to 12:00.
Now, I will generally just ride most things once per day, so re-riding isn't that important to me.
Of course, if the 4th FP thing rolls out and all that's available is Captain EO, it will obviously derail my plans. But I looked at the survey Disney sent with ideas of what would probably be available, and I think I have a good chance of cutting down my wait times to my preferred amount, just as I did with FP-.
I won't be maximizing the time saved per ride, but I'll be minimizing time spent in lines overall.
What I like about the additional FP+, is there is no right or wrong strategy. Using FP+s in the afternoon might work perfectly for some and using them in the morning will be great for others.