I believe the 400m gauge mod is to be used in conjuction with the SH3 Crush depth V2 mod at....
http://www.delraydepot.com/tt/sh3.htm.
In essence I used the crush depth mod to modifiy the max depth of the VIIC/42 (which we all know didn't exist but I use it instead of the XXI). You can use
www.uboat.net to ensure you have the correct max depths i.e. VIIC/42
http://www.uboat.net/types/viic-42.htm scroll to the bottom.
The crush mod also adjusts range whether surfaced or submerged.
Hope this helps...