I would assume a five based on what others are running, but I'm open to suggestions for starting points.

my 27 Fountain likes the P5X with the -3

I wish I had gone with the -3's. I went with the -2's and yes I have looked into installing extension boxes and -3's but I have decided to put the $ into a new boat. Just hoping to find one before spring.
Looking for a 28 - 30 Daytona with standard power. NO open bow. $60,000 to $70,000

A 5 blade is going to give you 5 -10 MPH at about 60 mph Cruising. Meaning less Rpm at cruise. You will probably run a 30- 32 on your boat. Tell me where your proshafts are, we have same hull, same year. Brett at B blades will rent you props for 25 each. That is the way to go prop testing. Also they are pretty good on what works
Chad