measure the cable pulled. a piece of tape around the cable
with a sharpie mark on it will do.

I have found just by the feel, unfortunately, adding a Top mount brake lever
into the cable of an aero brake lever, ends up as an adjustment compromise
because the leverage of the 2 levers are entirely different.

More for the lever on the end, less for the ones in the middle,
so somewhere between pulling the aero lever to the bar with little effect
and having the top mount lever grab the rim with almost no motion at all,
but with significant, but very careful, effort.