Does one stay locked in place? Unclear.
If you set C1=A1+B1 then, when you set a value for C1, A1 and B1 are each half of that value, even if they started off unbalanced. Does one stay locked in place? Unclear.
If you set C1=A1+B1 then, when you set a value for C1, A1 and B1 are each half of that value, even if they started off unbalanced.It also doesn't help that in the example, the expected outcome of 53.3333/46.6667 isn't even considered.
A1 = 1.0 // the scale, your variable
A2 = 6 * A1 // intermediate values
A3 = 8 * A1
A4 = A2 + A3 // the sum
Now update A4 (or any other cell!) and the scale (A1, the only variable) will update as you expect.