It's not exactly rocket science is it? The blasted leaflet Joe bangs on about was based primarily on forecasts done by the Treasury and the Institute for Fiscal Studies into the impact of Brexit on the economy. Osborne will have used those forecasts to project how public finances would be impacted, and then did outlined some measures the government could take in response to that fall in revenue. If I remember rightly, it wasn't just Osborne making the statement either, as he was sharing the stage with Alistair Darling so it had cross party support.
I believe they split the £30bn hit on public finances between spending cuts and tax rises. As is usually the case with these things, the poorer would likely have been effected more than the wealthier, which to be honest is exactly what will happen with Brexit anyway.