Bradford Werner has a very useful and details discussion on nail shape on his "This is Classical Guitar" website: https://www.thisisclassicalguitar.com/f ... al-guitar/
. You will see appropriate nail shape varies according to hand shape and finger orientation, and is very much "horses for courses". He also refers to three other videos on shaping the nail by Matthew McAllister, Thomas Viloteau, and Tatyana Ryzhkova. The first two advocate using sand paper over strings to get the initial ramp shape. The resulting shape between McAllister and Viloteau are quite different. I personally find using a medium grade sand paper over the strings is a good guide to getting the initial angle from which I can file the shape and polish the edges to get a good sound. David Jaggs also has useful guidance on nail shape and tone production (http://www.davidjaggs.com/digital-download.html
). You will find an introductory video on tone production by David Jaggs on YouTube, but you will have to pay for and download his video for the full details.