hi
how short is your hair? if your hair is long, half a cap full rogaine foam won't be enough to cover your scalp and most of it will just get wasted on your hair before it gets into your scalp. So if you have long hair, it might be better to get a liquid version?. Regarding application, I don't use the cap. I just rub it between my hands to liquidize and apply it to my scalp. It doesn't have to be half a cap full. You just need to experiment a bit to find the right amount.
I haven't heard about rogaine foam destroying hair line but I doubt it does. I mean if the foam version does ruin your hair line, the liquid version should do too. They both have minoxidil as the main ingredient. I guess unless you try it yourself, you will never know!!
Good luck!!