On saturday i have a violin concert. After the concert i will hand my parents my coming out letter and say "i want you to read this without me there. So im going to go upstairs while you read it"
It would definately work, the downside with letters is that you can't see their first reaction, aside from that I don't see any reason why this wouldn't be a good way to tell them.
This is a good way to avoid a direct confrontation which can sometimes lead to strong reactions and allows them to process it as they see fit. Indeed it also allows you to say something afterwards like "did you read my letter?" and "so what did you think?" if you so choose. So I think it's great.
I think that is a good idea If that what you wanna do I'm not even out to my parents yet and I have no idea how to come out to them or other members of my family so good luck and update us if you can.