Compression tester--theoretical question

[kerry]

I have a diesel compression tester with a variety of adapters. There are a number of glow plug hole adapters. Some of these adapters include the pencil tip section on the adapter. The adaptor that fits the 617 pencil plugs does not include the pencil tip length but stops where the pencil tip would normally begin.
How will this effect compression readings?
The combustion chamber is quite small. It seems to me that the pencil tip on the glow plug is quite large in comparison to the size of the combustion chamber. Will this additional area (normally taken up by the pencil tip), now available for the compression chamber, significantly lower the compression reading results?
Or, given the fact that the volume of the pencil tip compared to the full volume of the cylinder with the piston at bdc is relatively small mean that the change in final compression readings will only be negligibly smaller?