Length of a Craps Hand - recursive table?

What is the average number of rolls between Come Out rolls?

For exacly number of rolls in execl where r is the number of rolls 6/36*pow(27/36,r-2)*(9/36)+8/36*pow(26/36,r-2)*(10/36)+10/36*pow(25/36,r-2)*(11/36)
Works for all rolls exept the first one.
google docs spread sheet
Sample useing rolling a 4 on the come out roll(3/36). Rolls then can either make the point (3/36), 7 out (6/36) or be something else (27/36).
A 2 roll would be either 4,7 (3/36)*(6/36) or 4,4 (3/36)*(3/36)
A 3 roll 4,O,4 (3/36)*(27/36)*(3/36) or 4,O,7 (3/36)*(27/36)*(6/36)
Hope that helps. Ask any questions.

Thank you