JD: Thanks, but I think I may not have put it clearly enough! I was not talking about the calculation of the TL.
What I was referring to was colleagues in ATC who would argue that, because the minimum stack level in the LTMA (i.e. the lowest FL which gives AT LEAST 1000ft separation from ALT 6000') was FL80, you couldn't use FL70at all. But if I need to provide separation from traffic at FL80, then FL70 was fine. It just didn't also provide separation from ALT 6000'.
If you consider a QNH of 1012, then FL70 is only approx 970 ft above 6000ft (the maximum SID altitude in the LTMA) so is not available for use in the holding stacks, thus FL80 became the minimum. That meant that actually there was 1970ft separation between traffic at 6000ft on the SID and traffic in the hold. If I wanted to keep departing traffic climbing as much as possible I could climb it to FL70 (1000ft below the holding traffic) provided there was no confliction with other traffic underneath at ALT 6000ft. Some ATCs could not understand that logic...
Edited to say: Thanks anotherthing, I was busy typing when you posted your reply!