Ok well I ignored the ratio in my first post observations.
Turns out that max depth does not have a bearing on the boat taking damage.
The formula is a lot less complicated then that.
The point at which you start taking hidden damage is 70 % of crush depth listed as crash depth in the subs zone file.
Which means with no damage to the boat from other things like depth charges, bombs, etc, etc, then You should be able to list your red line in the max depth to 70 % of the crush depth and be ok.