Subtract running totals based on condition (M Code)

[Zaibass81]

Hello,

I'm looking for a m code solution that subtracts running totals and re-iterates calculation when a new FG Item is found, also to return last value from iteration to the beginning row 

[wdx223_Daniel]

NewStep=Table.FromRecords(List.Accumulate(Table.ToRecords(PreviousStepName),{{},[]},(x,y)=>let a=Record.FieldOrDefault(x{1},y[FG_Item],y[#"RM#QTY in WHS"]),b=a-y[#"RM Part# Monthly Consumtion"] in {x{0}&{y&[Remaining Qty=b]},x{1}&Record.AddField(y[FG_Item],b)}){0})

[Zaibass81]

Unfortunateley I am getting errors.

First one related to the null value converting to text

then if I am filling up FG_Item colunm to replce nulls i am getting another error that record wasn't found..