I'ld like to share my knowledge among those who play classical guitar. On the picture 1 you see fingernail drawing.

This is the right hand finger ie finger, which needs nails. For lefthanders: look to this picture through mirror. There are three letters:

1. Nailcorner closer to thumb. This corner must be filed out rather bravely. If this corner is too big, it gives to nail bigger resistance. Idea is, that finger contacts string with flesh at first and then there will be contact between string and nail. Rounded nail is not disturbing this process.
2. Nail's length. It's matter of taste largely. I like the rule, that I can see 1 mm of it when wieving them from opposite side, the imaginery side behind screen.
3. Outer nailcorner. This corner can be rather sharp when left alone as it grows. It must be rounded off. But not as much as A-corner. otherwise the playing length of nail will shorten and sound looses It's fullness. The sharpness of It is hearable distinctively.