Hi Shane,
You can dynamically choose all periods greater then a selected time period using a couple of select statements.
The first you'll need to return the TIMEID of the selected time period.
The second you'll need to return all members who's TIMEID is greater then the value returned in the first select statement.
You can then use all the values returned in the second select statement in your equation.
Let me know if you want any help on this.
Cheers,
Chris