I have the tip of my forefinger on the spine/tang and the flat of my thumb pad on the bottom side of the tang directly opposite my forefinger. My three fingers, middle, ring, and little finger, are lightly supporting the scales. The 1961 barber manual excerpt PDF in the SRP Library recommends practicing the flip without stropping the razor until you feel you've got proficiency at it. Then begin moving along the strop. Check out the PDF, the last few pages are on stropping. What deadrift said about snug scales is right on.

http://straightrazorpalace.com/srpwi...t_-_Honing.pdf