Baluster spacing
The count, and the exact mark for every baluster measured from the newel face.
Why a count is not enough
Most calculators give you a number of balusters and stop. The number is the easy half. Setting them out from one end at a fixed spacing leaves whatever is left over as a single oversized gap at the far end — and that gap is exactly where an inspector puts the sphere.
So this one divides the leftover evenly and gives you the centre-to-centre mark for each baluster from the newel face. Mark them all before you drill.
On a rake
On a pitched run the sphere is applied perpendicular to the rail, not vertically, so the horizontal spacing and the along-the-rail spacing differ. Enter the span measured along the handrail and the result is correct for the rake.