topdrop,
The TA, is the same, that's the point. TA and TL are based on local QNH but the min stack level in a TMA is based on one specific aerodromes QNH.
So for example in the Scottish TMA Edinburgh and Glasgow have a TA 0f 6,000 feet. The transition level is then calculated based on the local QNH at each airport and broadcast by ATIS. However min stack within the Scottish TMA is always based on the Glasgow QNH. Therefore at the boundary pressures where min stack level is changed, if the QNH at Edinburgh is lower than at Glasgow you could be told, for example, to descend to FL070 and then told the TL is FL075.