Their are multiple opinions on the use of Fastpasses for a one park day.
My opinion is to set them up for right after lunch and before dinner. My reasoning is that if you are at the park for rope drop (actually I recommend my clients be there 30 minutes prior, in case they open early-which they can and will do) to gt a jump start on the lines.
Once I get on the line for my third Fastpass or as soon as I get off it, I pull my phone out and set up my next Fastpass. Keep in mind to use your first 3 for the longer line rides as the further you get into the day the fewer Fastpass choices you have.
The most important thing is to get your Fastpasses as soon as you can, 30 or 60 days out, or if in the park then as soon as you use your 3rd Fastpass.
PLEASE use the Fastpass system set up by Disney to make your time there better and avoid standing in so may lines. Enjoy!!