You don't really need an anechoic room. I do it in a dead vocal booth and i use a flat calibrated cardioid mic as a reference mic. No point in using an omni mic as a reference for measuring a cardioid. I double check sometimes with an omni.
A lot of companies use small booths or chambers. Neumann uses also a small one for production run quality check. Large ones are used mostly for developing.
I am not really interested in what's going on below 500hz, condensers are mostly linear there unless the circuit isn't.
I use mostly 10ms iR window

Next best thing if you don't have anechoic chamber is free. You do it outside

If i am especially interested in the capsule measurement i don't use any headbasket, and i use a custom made body that doesn't affect the sound. The capsule is basically floating in the air free of all disturbances. I make sure there is no EMI, and if there is a tiny amount it's visible only at 50hz.