Karel, I could not found any original pictures of U-995 that you could see the flooding vents clearly in. However, I am sure there was no standard pattern of flooding vents for this Turm as every boat yard used a different pattern.
We can deduction that the spacing of the flooding vents are expected to be in groups of 500 mm as the internal framing was space every 500 mm. So you could go for two small vents every 500 mm, or one middle size vent every 500 mm.
I would go for 1 flooding vents per frame. So at 1:72 they are every 6.9 mm and the size are (100x60mm) so at 1:72 1.3x0.9mm. This is what I did for my drawing. You can see that I am talking about in my drawing below.
Simon
