See, some books will tell you "Knee length skirts are modest." And... I suppose that might be true... But they aren't practical. At all. If you sit down, you have to constantly worry about whether or not you are covered. If you try to run... you have, um, issues. Squatting, too, poses some problems. And on and on. Basically, you can't move. haha. (Okay, fine, that's an exaggeration...)
So! I've decided that long skirts are just more practical. (Provided, of course, that they are reasonably full. A tight skirt is obviously not practical OR modest. :P) You rarely have to worry about them covering you--they do it amply. (The only instance I can think of at this moment that they wouldn't is if you tried to hang upside down. ahem. haha!) You can move much more freely in long skirts!. Plus, they are more fun! ;)
And with shirts... they say that if your shirt is up to your pinky finger when you put your thumb on your clavicle (please tell me you know what/where that is!) it's modest. Well... generally speaking, skirts like that gape. Not pretty. So, for practicality's sake, (I'd rather not hold my shirt all the time...) I wear them at least right up to the clavicle*.
So, basically, it's better to just go all the way with modesty. Don't try to show as much as you can and still be "modest". If you do "go all the way with modesty", (aka Practical Modesty) you have much more freedom, are less self (and clothes!) conscious, and... you are showing your brothers and your God that you are willing to go that extra bit. :)
There you have it! Practical modesty. Just my personal view.. makes life simpler. And, hencely, more fun! :D
*Just so as we're clear, much as I hate to do this... clavicle = collar bone.