IR, we had this discussion several months ago here:
http://forums.irvinehousingblog.com/discussion/877
In it, I posted my calculations on GRM and how I came up with 200. Since that discussion, I see you made an improvement to your original GRM calculation (by adding in the benefit of principal repayment), although several other small changes canceled that out and you ended up with the same GRM of around 160.
I would say the errors in your calculation are primarily two-fold: 1) Missing the tax deduction for CA income tax at 9.3% and 2) Overestimating the reserve needed for repairs. Actually, now that I do the math, I'm not sure you have the tax savings right in that calculation even leaving out the state tax issue. $2166.67 + $416.67 = $2583.34 x 25% = $645.84, not $567.71.
The second issue, how much is a reasonable reserve for maintenance and repairs, is trickier. Several people in your thread wrote saying they thought $625/month was too high as well. In the first five years I've lived in my home (although, I will admit, the home was new when I moved in), I've spent $350 on repairs and maintenance (outside of something minor that was covered under home warranty). That's $5.83 a month. I know a neighbor who had a bit more problems than me and has spent approximately $2000, or $33.33/month. My parents, who moved into a home that was 40 years old at the time, and have lived there for 25 years, have spent approximately $70,000 in repairs/maintenance over the years, and they've had to pay for an entire roof replacement, multiple roof repairs, pool/pump repairs, garage work, plumbing, electrical, etc. That's $233.33/month. Oh, and their home is worth over $1M, not $0.5M.
So, I think if you use $250/month, maybe that would be more reasonable (still high in my opinion). If you put that into your equation, along with correcting the tax issue, you get:
$3503.27 - $886.09 - $361.31 + $312.50 = $2568.37
$500000/$2568.37 = 195
Anyway, the main reason I think the numbers the researchers came up with (218-240) are reasonable is that none of this takes into account several factors: 1) Over the long term, real estate appreciates. Yes, yes, I know it won't for a few years and it'll take a few more after that to make up the losses, but I didn't say you needed to buy now, and I mean over decades. 2) Owning is an inflation hedge. 3) The above calculations are on the *worst* month possible, namely, the first month of repayment. Over the months and years, interest decreases and principal repayments increase, so the GRM can only go *up* from 200.
But, I guess we'll all see over the next two years, eh?