I think it depends on the body type. I'm a bit of a mesomorph, and I couldn't get within 3 kgs. of that (even when I was a high school X-country runner with <5% body fat). Nowadays, there's absolutely no way ;(. It's probably more accurate for fast-and-young triathletes....

Joe Friel makes mention of
"Determine your ratio by dividing your weight in pounds by your height in inches. Competitive male triathletes are generally about 2.1 to 2.3 pounds per inch. High-performance women triathletes are usually in the range of 1.8 to 2.0. Men who exceed 2.5 pounds per inch and women above 2.3 are best advised to find flat race courses if the goal is to be competitive. Hilly courses favor lower-body-mass athletes. But high-mass athletes actually have an advantage on a flat bike course; all the more so if the wind is blowing. For example, the 200-pound athlete mentioned above comes in at 2.5 pounds per inch and races most competitively on flat to rolling courses."
He goes into a little more detail of this in his books which I don't have handy at the moment.

I'm about 2-3kg over that weight now, but it actually lines up almost exactly with what I think my ideal would be. Just haven't gotten there because of beer and french fries and such:).

for male triathletes this works pretty well:

0.90[50kg +2.3(inch over 5 feet)]

When considering body fat %, it is sometimes better to take a practical approach rather than a "less is better approach".

I would definitely recommend the book - it's a great read.

Joe Friel makes mention of
"Determine your ratio by dividing your weight in pounds by your height in inches. Competitive male triathletes are generally about 2.1 to 2.3 pounds per inch. High-performance women triathletes are usually in the range of 1.8 to 2.0. Men who exceed 2.5 pounds per inch and women above 2.3 are best advised to find flat race courses if the goal is to be competitive. Hilly courses favor lower-body-mass athletes. But high-mass athletes actually have an advantage on a flat bike course; all the more so if the wind is blowing. For example, the 200-pound athlete mentioned above comes in at 2.5 pounds per inch and races most competitively on flat to rolling courses."
He goes into a little more detail of this in his books which I don't have handy at the moment.

I'm about 2-3kg over that weight now, but it actually lines up almost exactly with what I think my ideal would be. Just haven't gotten there because of beer and french fries and such:).

for male triathletes this works pretty well:

0.90[50kg +2.3(inch over 5 feet)]

LOL there is NO WAY that would work for me. I am 5'11", 185lbs, 14% body fat. So I have about 25lbs of fat on my body. That formula puts my ideal weight at 150lbs, which would mean I would have to somehow lose 35lbs of body fat even though I only have 25 - maybe if I amputate a few limbs :)

170lbs would put me in the single digits for body fat, which is what I am aiming for right now. I'm just a big dude - I look more like a football player than a triathlete. Weirdly, despite my size, running is my strongest event.

LOL there is NO WAY that would work for me. I am 5'11", 185lbs, 14% body fat. So I have about 25lbs of fat on my body. That formula puts my ideal weight at 150lbs, which would mean I would have to somehow lose 35lbs of body fat even though I only have 25 - maybe if I amputate a few limbs :)

170lbs would put me in the single digits for body fat, which is what I am aiming for right now. I'm just a big dude - I look more like a football player than a triathlete. Weirdly, despite my size, running is my strongest event.

you (as do I at nearly the same size) have your answer. Lose muscle to get faster. Your FTP might go down and your top 100 scy might be a tick slower but probably not. Your run will rule enough to compensate and the only place you'll lose any real time is perhaps on the flats during the bike if you are currently going fast there. For some people the extra training required opens up a new level of fitness and the lost muscle doesn't matter.

Anyway, losing muscle is a lot slower process than losing fat. Hard on the body and it literally feels like you are starving verses just a reasonable fat -losing diet which just feels fine. Durning weight cutting periods (former lightweight rower) I started having trouble sleeping because I was so hungry and training so hard. Slow to recover and very easy to get sick during that time. A good friend and teammate had liver and kidney problems as a result of cutting too hard he suspects...still an issue 10 years later for him. Proceed with caution. Me I am going for the last easy 15 lbs of fat off to get back into the single digits bf then slow multi year plan to carve another 10-15 lbs off.

Indeed proceed with caution. I understand what you are saying and I don't disagree with it but folks need to remember that heart is a muscle and your body doesn't pick and choose where it catabolizes. Over time, this type of weight loss could potentially lead to problems with cardiac function.