In theory what you wrote sounds good, but it doesn't match the actual payoffs that show the P6 vs P5 is a lot closer than you're thinking. I still believe the P6 is the better play. Here are the payoffs each day since it began. Keep in mind that the buy in for the P6 (20 cents) is 60% cheaper than the P5 (50 cents) for 1 more race. The cheaper price gives you more opportunities to spread especially if you like fewer than 3 horses in the 1st leg of the P6. Based on your theory, shouldn't the P5 be paying more since it costs 2.5 more than the P6 and has a better takeout? The payoffs don't match your theory. For today's card, I would say many could have singled the 1st leg of the 6 and have more options to spread in later races. Any time the 1st leg can be had with a single, P6 is the way to go.

8/7 P5 $644 P6 $621
8/8 P5 $37,191 P6 $3,308 (5/6)
8/9 P5 $13,804 P6 $9,950
8/10 P5 $486 P6 $999
8/11 P5 $4,451 P6 $1,997
8/14 P5 $13,044 P6 $25,145
8/15 P5 $739 P6 $706
8/16 P5 $676 P6 $968
8/17 P5 $1,855 P6 $4,949
8/18 P5 $2,565 P6 $12,547 (mandatory payoff)
8/21 P5 $4,515 P6 $37,064
8/22 P5 $430 P6 $3,825
8/23 P5 $2,124 P6 $1,907