How to reference value from a list in split function

If you split it in Rows not in columns, then you just need to specify the PositionList.

Select your columns and while splitting choose Rows in place of Column under Advanced. Put any dummy value say 3 as position. Once split done, in formula bar, replace this 3 with name PositionList.

You will get this kind of split

= Table.ExpandListColumn(Table.TransformColumns(Source, {{"Text", Splitter.SplitTextByPositions(PositionList)}}), "Text")

We can make formula for vertical split as well but in my view this much complexity is not needed and also for Power BI, it is longer table rather than wider table which is needed.