People under different circumstances, though, may lose more or less LBM than that. For instance, a person checking in at 25% body fat and dieting down to 15% body fat may lose only about 15% LBM and 85% fat. Generally, the more fat you're carrying the greater percentage of fat you'll lose as you lose weight. Likewise, the leaner you are the more LBM you're going to lose as you diet. A competitive bodybuilder might lose up to 50% LBM in his last bid to get down to extremely lean levels in the 4-5% body fat range. Rank beginners might actually be able to gain a little muscle as they drop body fat... as long as their diets aren't too severe, last too long or they're not super-lean to start. Past the beginners stages though, that's highly unlikely to happen. Generally speaking, the bigger your calorie deficit is, the faster you lose weight, and the more total weight you lose, the more lean body mass you'll lose.
Notice that I keep referring to LBM and not just muscle. Why is that? The simple fact is that lean body mass is composed of much more than just skeletal muscle. Lean body mass means your skeleton, skin, organs, contents of your stomach, glycogen in the muscles and yes, your skeletal muscles. Even if you did manage to hold on to 100% of your skeletal muscle mass as you lost weight you’d still lose lean body mass because of these other factors… which is why even the best natural bodybuilder will lose LBM as he/she diets down.
Final Weight = (Current LBM – 0.15 x Current Body Weight) / ( [1 - Desired %bf/100] – 0.15 )
But bear in mind that this more optimistic rate of LBM loss applies to people starting off fatter than a typical off-season natural bodybuilder and not dieting down to very lean levels or dropping weight too quickly (say roughly 1 pound a week or less). So don’t think that as an experienced trainee you can go all the way from 15% down to 6% body fat and lose only 15% LBM in the process. Unless you are very gifted and/or have reliable past experience or other special circumstances to indicate otherwise (i.e. coming back after a training layoff or start taking steroids), that isn't likely - stick to the 3:1 "rule".
Conversely, many less gifted bodybuilders/athletes will lose more than 25% lean body mass when dieting. This may be due to a multitude of factors from over-dieting to overtraining to genetics, but 36% is an average number based on less gifted "clients" I've monitored over the years... you might even say that this 36% rate is more typical of the average person than the 25% rate that shows up often in competitive bodybuilders. I myself tend to be in the 25% to 36% range depending on how quickly I lose weight.
If a person crash dieted and did no weight training or exercise whatsoever during the diet, up to 50% of the weight they lose may be lean body mass. This has been observed repeatedly in research environments.
The take home here is that, no matter how hard you may try, you're going to lose some lean body mass during your diet - unless you are a rank beginner to weight training and just losing a few pounds through a mild diet and exercise program. Similarly, if you're returning to intensive weight training after a layoff you may be able to regain some lean body mass that you previously had (i.e. "muscle memory") as you lose some fat, but that situation won't last long and the diet would have to be mild. Otherwise, you're going to lose somewhere in the vicinity of 15% to 50% lean body mass as you lose weight, depending on how much weight you lose and how fast you lose it (i.e. how large your calorie deficit is).
An online calculator that does these calculations is here: Final Body Weight Calculator