Ok,
Making it simple for you.
Ignore temp (T) as we'll assume for our rough and ready calc the temperatures aren't really going to change.
So you've got a 12L with 100bar of 50% and you're going to air top it. What mix will you get?
You've got P = 100bar (50bar O2, 50bar N2 as it's 50%) and V=12L - Remember its Partial pressures that go to make the whole P
So you've got 50 x 12L of O2 = 600L of O2.
Now you're going to top off with air to 232bar.
Thats 232-100 = 132bar of 21/0/79 or 132 x .21 x 12 = 332.64L of O2.
So my final mix is 600L add 332.64L (932.64L) of O2 out of 12 x 232bar (2784L)
932.64/2784 = 33.5% O2.
Can you see all we've done there is P1V1+P2V2 = PV
He is just as simple you just run through with three gasses.
You can work it backwards to work out what O2 you need to put in a tank to mix your own Nitrox and it can all be done on a phones calculator, or if you can, your head.
The nice thing with doing all that in your head is, if the mix comes out high, you know he's not filled the tank to 232bar and if low, you've got an overfill.
Hope I've got my sh!t in order
B