Oops, I already forgot that you have step between mid and tweeter. Advantages of common (average) elevation for M+T are partly lost due to different Z. I would measure M and T at individual elevation.
Another option is to measure all drivers at 1000 mm from W+M baffle. Then you can measure M+T at middle elevation. Z mm should be 0 mm for all drivers, because distance differences are included in phase responses. Standard 1/48 oct. frequency resolution is able to handle about 25 cm distance differences with the same mic position up to 20 kHz. High resolution export with Convert IR-FR tool is limitless i.e. phase response is able to transfer any measurable distance.
Another option is to measure all drivers at 1000 mm from W+M baffle. Then you can measure M+T at middle elevation. Z mm should be 0 mm for all drivers, because distance differences are included in phase responses. Standard 1/48 oct. frequency resolution is able to handle about 25 cm distance differences with the same mic position up to 20 kHz. High resolution export with Convert IR-FR tool is limitless i.e. phase response is able to transfer any measurable distance.