How can I use Scid to figure how I should divide study time for each opening line? I have a database (Scid) that is mostly made up of games downloaded from TWIC. I know my rating (low 2000s).

As an example, I want to play the Classical Sicilian as black. It can be reached either as 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 Nc6, or with 2...Nc6 and 5...d6, my choice.

Of course, White has many other options as well, and I want to divide my time proportionally. If I reach the Classical 50% of the time (after 1.e4), I should devote 50% of my 1.e4 black training time to it. Maybe 20% of the time 3.Bb5 is played, et cetera.

What's the best way to figure out such percentages correctly?
I would recommend an iterative approach to study an opening system. Use it in a game, then analyze the opening with a large opening tree, book and engine. For example, you can use the Chessbase mobile app or the corresponding Chessbase Online site. You can record your conclusions in your own opening database file, e.g. using Scid.

By continuously using and improving an opening system, learning and winning will go hand in hand. By immediately analyzing the opening after every played game (online as well as over the board), you will improve your opening skills step by step.