I don't have Excel available at moment, so I can't see what you have in your attached workbook, but on fly:
Add a column, where the number of table row is calculated. Something like =ROW()-1,
where the number is number of header row.
Use SUMIFS() to calculate the sum for all part numbers equal to current one, with table row less than current one.
My advice is, use Defined Table instead regular one - so all formulas will be updated atumatically whenever a row is added. Then the formulas will be like
Code:
row number:
= ROW() - ROW(YourTableName[[#Headers],[AnyColumnName]])
running total:
=IF(OR([@AmountName]="",[@PartNumberName]=""),"",SUMIFS([AmountName],[PartNumberName],[@PartNumberName],[RowNumberName],"<" & [@RowNumberName]))