I'm not exactly understanding why you want to feed the panel from the
existing FPE panel, if you're moving the service to the new panel, unless
this is just for a temporary setup.

Yes, it's only temporary. Rather than replacing everything at once,
it will let me rewire the different circuits as I can, and would
ensure minimal downtime when it comes time to move the service to the
new box. The old FPE is history after that.

Thanks for the response.

-Charlie