Does the BB axle (spindle) lengths 1:1 compare for square taper and octalink?
No. And not only that, if you were to stay with square taper cranks you would find that the BB length for one crank does not necessarily match the BB length for another crank.

In other words, if you use a 103mm square spindle bottom bracket A with crank B, it does not mean you would use a 103mm bottom bracket C with crank D, regardless of whether bottom bracket C is square or octalink.