I generally aim for a locked 25 (usually achieveable).
The problem with locking the framerate is that it lowers the performance in general. Meaning: in scenes where you would usually get 30fps or even more you only get 15-20 when locking to 25 fps, for example.
But if 20FPS or 25FPS is offering a smooth flight, then you would have lost NO performance. You look at the numer of frames as a measure of performance and in so doing, are always trying to 'improve' your performance. I and others, look at the smoothness of the sim. We learned long ago that chasing FPS is an endless quest. I got the sim running smooth and never (rarely ever) look at the FPS because that number is meaningless, unless I want to brag to someone what my fps is.
Let me rephrase that: in scenes where I get a smooth impression of flying when set to unlimited, I get a jerky and stuttery impression of flying when locking the framerate regardless of the actual fps number. I just used the framerate numbers to put the subjective impression of visual graphics in text form that others can read objectivly, in order to document the loss of performance (and a lower average performance means that it hits the unplayable bottom in peak situations). I never use the FPS counter when flying. But when I notice stutters and jerky performance I sometime check the fps counter in order to have empirical data that proves that I'm not just imagining things but that there actually is a performance loss.
All I wanted to say is this: at autogen heavy locations like big cities I get smooth flying when I set the framerate to unlimited. But when I lock the framerate I get much, much, much worse performance, close to the unplayable. Or in other words: the performance with locked framerate is far worse than it should be, because it stays way (and noticeably!) below the locked framerate even though the performances surpasses that of the locked framerate when set to unlimited. The logical thing to expect would be that the performance levels out at exactly the locked framerate, but it doesn't. This is something that should be kept in mind when locking the framerate. It has an actual effect on the quality of flying not just on the numbers. I hope you understand now what I was on about. It was about a huge and noticable loss of performance when locking the framerate and not just about feeble bragging numbers.