Their conclusion is that lowering taxes will increase revenue, the money that would have gone to the government stimulates growth in the private sector, but not to the extent that it makes up from the revenue lost from the tax cut. Some of the tax revenue lost will come back to the government in taxes on new growth, but not enough to make up for the cut. This is what they call a robust finding. By How much will total revenue increase? Fuck it, people argue about that endlessly, anybody willing to give you a straight up number is probably bullshitting you or themselves, it's probably different in every situation, while they might be able to nail it down for past occurrences, predicting how the whole thing will shake out next time, or tomorrow is probably impossible.