...for example if I need three minutes I'll probably start giving the take-off clearance at 2min30s as by the time you've read it back and rolled, the three minutes will have elapsed
The problem is that in callum's situation there is probably no vortex wake spacing requirement (certainly not a clearly applicable one from the info we have). Perhaps the best message that we can gain from callum's experience is that you can meet vortex turbulence anywhere (even if the minimum spacing requirements have been applied by ATC). Hence the need to be aware of the conditions in which vortices are generated and also what cases them to dissipate - or not - more quickly.