You're not calculating a long drop such as used for executions. And if you are, don't, ease in full suspension is better and less technical than long drop.
For full suspension it's literally the length you required from the anchor point with enough for a noose.
Learn to tie a good anchor knot such as a bowline. The slipknot noose takes less than 5 seconds.
Simply cut a rope to the length you require or have a long tail at the anchor end.
The length of rope makes no difference at all once it's "in use".
The best resource here is Evelyn's Guide:
Who are you? Why should I listen to you? Can I trust you? I am a suicidal woman that has been studying about hanging for more than five years. I have attempted suicide by hanging a few times in the past, and I plan on dying by full suspension hanging. I have studied about hanging from various...
sanctioned-suicide.net