Each state will calculate them differently, for example, New Jersey offers Excel spreadsheets to show exactly how they calculate it.

> The calculators below are for use by administrative agents and developers of affordable housing to determine the initial sales prices and rents for affordable units in compliance with the Uniform Housing Affordability Controls (UHAC).

https://www.nj.gov/dca/divisions/lps/hss/calculators.html

Rent/Mortgage under or equal to 28% net income.

That's hardly a consensus nor even enough aggregate consensus to serve as a rule of thumb.