Start with the clear width between posts. Equal spacing means the gaps at both ends match every gap between balusters, rather than leaving a large final gap.
Set the maximum clear gap you want to stay under, then use the calculator to choose the smallest whole baluster count that works. Do not narrow or modify balusters just to force a result.
Check the layout
Dry-fit one opening before cutting or fastening material. Product instructions and local rules can differ from a planning assumption.