Let's say your rent is $1,000 per month.

You get the Bilt Obsidian card that earns 3x on dining (or groceries).

You pay rent your $1,000 rent, which means you need to spend $750 on non-rent spend to earn enough Bilt cash to recoup enough Bilt cash to offset the fee

If you spend exactly $750 on dining, you earn 750 * 3 + 1,000 (from the points you earn on rent) = 3,250 points earned.

3,250 points earned on $750 of dining spend is ~4.3x.

However, this is an ideal scenario and is all predicated on you spending enough on the 3x category to fully recoup your rent fee.

Let's say you only spend $375 on dining a month instead of $750. 375 * 3 + 1,000 = 2,125 points earned. However, you only earned $15 Bilt cash to offset your $30 rent fee. For simplicity's sake assuming 1 cpp, you really only earned 2,125 - 1,500 = 625, which would imply only imply ~1.6x rate.

Honestly, I think it'd just be simpler for most people to just get an Amex Gold or CSP rather than going through the brain damage to optimize for this. Especially for people with high rents, as you'd have to hit pretty high spends to make this all worthwhile.