Industrial command ship yields

You don’t fly an industrial command ship for it’s personal yield, you fly one to boost the yield of other ships in the fleet flown by your friends and/or your own alts.

Some numbers I got from quick simulations in Pyfa:
(I assumed maximum skills, T2 equipment and mindlink. The numbers will be different than you have.)

Max ore yield from Porpoise with industrial core, 5 mining drone IIs and drone rigs: 14.9m3/s

Max ore yield from Retriever with T2 strip miners type B crystals, unboosted: 30.7m3/s
Max ore yield from Hulk, same setup: 50.2m3/s

Max ore yield same Retriever, boosted by Porpoise: 51.7m3/s (of which +21.0m3/s because of Porpoise)
Max ore yield same Hulk, boosted by Porpoise: 84.6m3/s (of which +34.4m3/s because of Porpoise)

As you can see, the yield of the Porpoise is low, and is less than the boost it gives to a single other barge. The yield of the Porpoise itself is even less than half the bonus the Porpoise gives to a single Hulk next to it.

For the mining yield of the Porpoise I wouldn’t use one. But, from the numbers you can see that a Porpoise + boosted Retriever mines more than two unboosted Retrievers. And a Porpoise + Hulk mines about as much as two unboosted Hulks.

So, for two characters it already breaks even to put one of the two in a mining command ship. If you have three or more ships in your fleet, the fleet as a whole massively benefits from having one of the characters in a mining command ship, from the added yield alone.

Don’t fly the Porpoise to mine ore, use it to boost your mining fleet. I probably wouldn’t even bother using drone mining rigs on it.

As to how you ‘tax’ the mining fleet, I guess that’s up to you. Some players with multiple characters won’t mind putting one in a mining command ship for the convenience and extra ore it brings. If no one is willing to do so, perhaps talk to your friends what a fair tax is.