While order of calculation may be part of the issue here, I believe the root cause is that you can't refer to DTS members in a member formula. They're "virtual" members. The only way to get a formula to apply specific logic to a DTS member is via "exclusion" in an IF statement.
Here's how it works - in your formula you have conditional logic that says if you're one of these time period members (and this logic must include ALL time period members) . . then execute some logic. Then at the end you have an ELSE statement. It is in this ELSE section (because you have already excluded all "real" time periods) that you can include logic to be applied to DTS members. But here's the catch . . . you can't differentiate between different types of DTS members. So a YTD member would get the same treatment as a QTD member, and so on.
Because you cannot differentiate between different flavors of DTS members in this logic, this solution typically doesn't cut it. But beyond that, you're out of luck. You can't reference a DTS member in a formula.
Hopefully my rambling post makes sense and I understood the question correctly.