Please start explaining from the basics:
- Is your BPC model using Periodic or YTD storage?
- Financial type model?
- Do you want this calculated for balance sheet accounts AST or LEQ type? And also P&L accounts EXP and INC types?
- Why exactly do you want to use a custom measure?
- Did you consider different use cases of the flow (subtables dimension)?
- Is it purely a reporting requirement for your customer to report on multiple years? How many reports like this are needed?
- Did you consider using just excel logic to do the same?