What is the best method of OPEN TO BUY?

To calculate OTB below things to be taken in consideration.

1. Sale target for the FY for existing + new stores (if expansion is in plan)

2. Liquidation/Sale - If planned

3. Opening stock for consecutive months/year

The calculation will be as follows:

(Planned Sales + Planned Markdowns + Planned EOM Inventory) - (Actual BOM Inventory)).

The BOM inventory should be as close to actual as possible.

OTB = Planned Sales + Planned Markdowns + Planned EOM Inventory - Planned BOM Inventory - On Order

I believe doing a bottom to up approach is better , to calculate from forward weeks cover .