Big caveat: as I understand it...

Calculation mode is stored as a per-workbook setting, so if you set calculation to manual all the open workbooks will be set to manual - if you then save your workbook, close it and set Excel back to automatic calculation, your workbook should open as manual calc (this is easy to test...) I learned this through many joyous hours of confusion with workbooks that one of my favourite esteemed colleagues used to send to me swtiched to manual calculation.

An alternative would be to set calculation to manual on the open event of the particular workbook - this may be more robust.

Having made these suggestions, I would never recommend manual calculation and would suggest there may be other ways of performing your calculations that are less-memory intensive, so you don't need to use it.

HTH