Just out the curiosity, is there any mention in any of the Traveller literature how much the Imperium taxes it's member worlds? I'm wondering if this 3% is more or less in line with the OTU.

I would imagine that in the OTU, there probably is no fixed rate, so everything is open to negotiation between the Imperium and the individual planetary government, with the world's noble probably playing chief negotiator for one side or the other depending on how he or she came to power.