Thank you for your responses. I tried guessing where the sides of the big and little toes are and working it that way. Then I tried your recommendation of just doing them all at the center of toes (except for inline decreases). Neither one seemed to look right because I kept getting a point at the center of the toes.
So, I went back and adjusted the sole pattern and decreased the no. of double stitches at the top of the sole (replaced 2 - 2 dc in st for center 3 sts, with 1 dc in in center 3 sts).
I have just started on the Upper Section. I plan to just do most of the decreases in the center with these exceptions:
Round 5 - "Decrease 1 st at each side of instep"; I will eyeball the center sides of slipper and make decreases there as you suggested.
Row 7: I will guess where the big toe begins and make 2 decreases, then guess where little toe ends and do 3 decreases. Hopefully this will shape the top of the shoe a bit. On the 2nd slipper these sides will have to be reversed (so I end up with a right and left slipper).
I will let you know how it goes after I get past that point.