How much vacuum is a matter of opinion
The vacuum used for aluminium coating varies widely and the pressure is often not a constant throughout the process. The system pressure before the process starts and the acceptable pressure during the process too is always a matter of opinion, sometimes backed up by historical results. If a given base pressure and deposition pressure was used and the resultant material was able to be sold into a particular market or to a particular customer, then it may become the required conditions for the process for that product. A different supplier of the same material may have a different experience and have a different base and operating pressure requirement.
The pressure in either system will depend on the same things such as pumping, total gas load and deposition rate, and changing any of these will result in a different balance that make the process viable. Bigger pumps do not guarantee a lower base pressure if the total gas load is also higher. The system leaks, incoming roll of film, any plasma treatment, any gas injection for web cooling and system cleanliness can all affect the total gas load. The incoming roll of film and the amount of moisture absorbed in the stray deposition on the shields may vary because of the seasons and local humidity level. So, even on the same system, the pressure will vary over months depending on the season or from run to run depending on the frequency of cleaning. Leakage from any gas injected between the web and drum will depend on the winding speed and web tension as well as the feed rate.