The base would be a tangent to a circle with centre at threshold, the turn to base being started abeam the threshold.
So the base is actually a much more constant distance to the runway than your normal circuit.
If you think about aiming points and such, this is much easier to judge and adjust. Just a slight modification of a spiral where your aiming point would be completely constant, hence, constant aspect approach.
Ah. Yes, now I understand what you are saying, once you say start the turn abeam the threshold. We call that a PTU here, prise de terrain en U. But there is no need to work out the angles, it is simply a U turn, half a circle with bank angle varying to adjust for the wind. Not an unusual technique when teaching proper stick and rudder flying and, dare I mention it, judgement.