I arrive there by deduction. See this US DOJ page for their summary.
https://www.justice.gov/enrd/us-v-washington

I would think that if alternate sources of income were part of the equation, it would say so, wouldn't it? Something like:

"Up to 50%, but no more than is necessary to provide Indians with a livelihood--that is to say a moderate living, inclusive of all other sources of potential contribution to their said livelihood"...

Again, I'm no lawyer, so take it as my opinion.