Although you'd think you could reference the row above with something like this: =INDEX(table.column,ROW()-1) it gives a circular reference error since it's referencing the same value, even though from another row.

When I've tried to get around this by doing an additional column where column 3 sums columns 1 and 2, and column 2 gets the value of column 3 from the prior row, it also gives a circular reference.

The only way I've personally found to do a running total like this was to have a 3rd column (which can be hidden) that calculates the row number (   =ROW()   ).  Then for the subtotal column, do a SUMIF that sums the value column when the 3rd column is less than or equal to the current row (   =SUMIFS(table_variable.value_column,table_variable.row_number_column,"<="&ROW())   ).  The only issue I end up having with this is that when I delete a row, it doesn't recalculate the row numbers in the 3rd column.  I get around that by disabling the ability to delete rows on the table (usually via CSS).  It's possible that issue with the row number formula not updating was fixed at some point (I'm still on version 10.2.1 in Forms).