According to the US government's definition[0, 1], affordable housing is less than 30% of income. Which type of income it is is up for debate but it seems the general consensus is gross income, though I can't find an authoritative source on that. If it
is gross income, then it stands to reason that 77% of net income may actually be fairly close to 40% gross income. And, while 33% above the affordability limit (30% of gross income), it's not terribly outside "a little higher than sustainable levels" -- though that's a very subjective phrase.
[0] https://www.huduser.gov/portal/pdredge/pdr-edge-featd-articl...
[1] https://www.hud.gov/program_offices/comm_planning/affordable...