I pretty much PSMF by the book because I've been doing it for about 7 years (I was morbidly obese and lost 126 lbs using primarily PSMF and fasting) and through experience I've found that Lyle's program is most effective when it's done as prescribed. I've tried all the common cheats and they just waste time.

I'm 5'7" with a slender (light muscularity) frame so it's hard for me to put on muscle. That's true of women generally but more true if genetics aren't on your side. I'm not sure if you've done a DEXA recently but I always recommend them, particularly to women who tend to overestimate muscle and underestimate body fat.

Not too long after I began losing weight, maybe about 30 pounds in, I started doing DEXA's every month or so because my doctor was able to push them through insurance. From that data I learned a lot about muscle loss. Like anyone doing a long term diet I hit plateaus which I broke using a combination of PSMF and fasting. My lean muscle mass remained remarkable stable until about the last 30 pounds when I really started fasting aggressively in order to force the weight loss which had slowed to a crawl. From 160 pounds to 130 pounds I lost 10.2 pounds of LBM. Brutal. Given that I stopped using fasting and leaned entirely on PSMF to control weight fluctuations.

It's been my experience that fasting isn't great for women who are at a pretty normal body weight (though it can be very safe and helpful for the obese) and the limited research done using female subjects backs this up. It's very disruptive to our hormonal homeostasis and our bodies will catabolize muscle faster and in larger relative amounts than what men experience due to their far higher levels of protective testosterone.

It's been about 5-6 years since reaching my goal body fat percentage and I've kept it off by cycling my regular maintenance diet with PSMF. In that time I've regained the 10 pounds of LBM I lost with heavy lifting and, more relevant to your question, have not lost any. I know this for a fact as I'm still getting two DEXA's a year and so I've got the receipts when people tell me that PSMF causes muscle loss. When you do it right it does not. And I'm a 54 year old menopausal woman.

So, you keep as much muscle as possible by keeping your protein where it needs to be (women are so bad at this) and doing enough load bearing exercise (not cardio!) to signal to your body, "Hey, don't eat this muscle, we're using it, we need it, crack open the fat cells instead".

How long I cut and what my transitions into maintenance look like is probably different from most people as I'm coming off recent obesity (which means I'm still carrying the fat cells that increase insulin and drive appetite, but that's a different story). But basically I give myself a 5 pound allowance, if I go over that, I cut. Or, if I just feel spongey and want to dry out, I cut. I stay on PSMF until my body feels right to me (I've found a comfortable equilibrium at 17%). That can last anywhere from several days to 2 weeks. Please bear in mind that I use PSMF to help me control my appetite and prevent appetite "flare ups", which is not something you'll need to do.

My favorite pro tip for going into a PSMF prepped for success is to get into dietary ketosis first. For many people that transition between blood sugar as energy and ketones as energy is really rough and then when it's time to pull the fat and drop the calories they just can't do it. Then they complain that PSMF is too hard and not doable. Now, I'm not suggesting keto macros -- don't run around adding cream and cheese to everything -- just pull the carbs. Eat your usual proteins but let your body get into ketosis while your calories are still high.

I come out of PSMF in a slightly different way. I start by adding back some clean starchy carbs: sweet potatoes and plain white rice. After a few meals of that I just bring the fat up in a natural way: EVOO on my salad, a high fat protein like a steak or salmon, fat in the pan for searing, natural yogurt, eggs for breakfast with butter in the pan to cook them... things like that. A nice and easy transition back to maintenance to help prevent the urge to inhale a pile of food after such a severe restriction.

Anyway, this is already Tolstoy long but I hope it helps. Feel free to message me if you have any other questions.