phase noise cascade

If I derive a composite phase noise requirement for a Tx chain, how should I divide this requirement with IF LO and RF LO...? Any rule of thumbs or references where equations, solutions would be available...

Thank you in advance

Noise is probably uncorrelated so you should be able to just sum the normalized noise levels at each frequency; if you have two sources of -100 dBc/Hz then total would be -97 dBc/Hz. Or if you have -100 dBc/Hz and -90 dBc/Hz you will get -89.6 dBc/Hz.

I used AWR Visual System Simulator to confirm it.