1. It uses a two-bedroom home as a baseline instead of a one-bedroom. While I understand that may be interesting for historical reasons, it's not a practical metric for modern considerations. If we want to talk about housing affordability, it's more honest to discuss one-bedroom apartments for single earners. We could also talk about two-bedroom apartments for dual-income earners.
2. The 30% pre-tax rent metric doesn't make sense to me at all. Why would that be invariant across income levels, and across states, given both graduated taxation and the vast differences in both income tax and sales tax across different states? In California, 30% pre-tax works might work out to more than 40-45% post-tax, depending on your income, and the remainder doesn't translate well to purchasing power given varying state sales taxes.
I'm not saying that affordable housing is not a problem, because it clearly is. Just that, to address the problem honestly, we need to use better metrics to figure out what a fair solution is.