There is an inherent flaw in the taxation concept - that is every world would need to convert it's local currency and economy to Imperial Credits. A poor world (say TL5) economy would be quite small in IMP Cr terms compared to a moderately populated higher TL world (say 12 - the Imperial average).
In this case you would very much have worlds that paid a pittance in taxes compared to more industrial and technical worlds. Even converting pure GDP won't solve the issue, as PPP (purchasing parity power) is a more accurate model of what the 'average' citizen can buy a serving of ramen for locally. And as we see in real-life (TM), imposing import and export tariffs are messy and rarely equate to the totals that are claimed.
This idea is terribly complicated and the game already butchers the economic model. Because of all the challenges I just ignore it completely. The Imperium gets its money in a magical way, it spends it in a magical way, and the running of the government is magical. It simplifies a lot of things and leaves more time to try and rationalize things that have at least a potential of being explained away in a reasonable manner.
They did that in an earlier version and it was a nightmare and did not usually add anything to the story. Easier to handwave it a bit.