1/4 of your take-home is a good formula. Remember that a portion of that $1500 is the principal/equity you're building, though, so should be counted as savings rather than as part of the comparison with rent. But that's a big difference: $825 vs $1500. If you rented something more like what you wanted to buy, what would you pay?
It might be helpful to think through more of your FIRE plans, too. Is your goal to get to FI as soon as possible? Once you're FI, what will you do (ie would you stay put or move somewhere else)?
Let's say you're 10 yrs out from FIRE. If you save the difference between renting and buying, earning 5% (or whatever number you're comfortable with) in the market, what do you come up with? If you were to pay off the house in 10 yrs instead, what does your FI picture look like? What's the appreciation like in your area, on average? If you sold in 10 years, where would that put you?
All of this is very location dependent, obviously. Someone in a hot market (San Franciso, or Vancouver) may well make more on their house than in the stock market, and then cash out to live in a lower COL area. In a slower area, appreciation can be negligible, but if that's where you want to stay after you've retired, then it can still be the right decision.
