How to Build a Cheap and Easy Measurement Rig

'Trace Arithmetic' never gives me the result I expect either (averaging, too), so if you're doing something wrong you're not the only one.

It's more reliable to export the measurements and manipulate them in Excel, then re-import them. File | Export | Export measurement as text (you can set the delimiter at the bottom of the menu), play around keeping the same file format, then File | Import | Import frequency response. Doing it this way, you can do things like making one measurement flat so you can see the results of mods or other headphones relative to that, which will eliminate many of the vagaries of your particular measurement setup.

It won't make much of a difference to your 800 measurements, but one thing affecting them will be that your mic isn't calibrated. @atomicbob has said that many 6mm capsule mics need a correction similar to the one shown in the graph and table at the bottom of this post; applying that as a generic correction will boost the mid-treble a bit. See here in the REW help for the file format.

Not absolutely necessary, but you could also calibrate SPL using the Virtual Sound Level Meter and a calibration file. Links and procedure are here.

Edit: Actually the datasheet for your mic suggests the response typical for that model doesn't deviate much from flat until beyond 10kHz. It'd be better to fake a calibration file from that than to use atomicbob's generic one.