how to choose components for boost converter
This is not an answer to your question, but it has implicatipons on your design (and I can't put pictures in a comment).

A 74141 is not a transistor, it is a decoder + open-collector driver. The 25 mA drawn by the 74141 is from its VCC supply. (last row in the table)



The VCC is ~ 5V, so you don't need to design your 120V to supply it.



Your load on the 120V will be pretty much only what your Nixies draw.

PS designing with a nice capacitor charged to 120V is not something I would recommend to a beginner. Or are you in for a Darwin award?