As you can buy miles for ~2.5 cents/mile on many programs (limited on most, apparently unlimited on AS), that is the upper bound of the value you should use.

I use this even though on an actual ticket cost basis I usually get 10+cents/mile (premium cabin TATL or Transpac).

Then, you need to add in a discount factor to compensate for spending money now vs in the future. Generally, the appropriate discount rate would be the interest rate of the highest interest debt you have.

Then you also need to figure in the miles that you are forgoing by no flying a paid fare.

Add all that together, and most times 2 cents/mile is good round number to use.