Water pump impeller clearance

I finally got around to replacing the water pump seal and drive shaft etc. 

I have all the required tools and the Maintenance Manual Heavy (MMH) states: The spacing WP01 (impeller clearance) is automatically established with insertion jig part no. 877258.

The problem is, I have replaced the seal twice using this tool and both times I can only acheive .007" clearance and not the minimum of .012" wich is a bit tighter than before I touched anything (previously .010")

I still have a small amount of Axial (End) play as before and confirmed this play is the gap between the drive gear and the housing. There was no visible wear in the housing or the gear but also the manual doesn't specify how much gap is acceptable between the gear and housing. I have checked on 3 seperate engines now and they all have a similar amout of Axial/End play as mine so I conclude this is normal.

I am now wondering if the tool (jig part no. 877258) doesn't have the correct dimensions. Given I have done this job twice now and got the same result both times, I think it would be possible to either make a 1mm thick water pump gasket or perhaps replace the washer shim behind the impellar with a thinner one (would probably have to machine one or modify the old one to suit) so that I can obtain the correct clearance. 

I am open to other ideas about what might be causing the lack of clearance and what to do about it if anything.