Mark.
The best Pictures of the Kingstons and vent to my mind are those on plate 28. As can seen they give a good opening with a fairly low restriction. Thus I think you can use the full floodarea may be - 5 to 6%. As you know there are normally two ways of diving. The conventional way and the wartime crashdiving the latter require a well trained crew. Conventional dive is the most common. The boat is in a E-motor mode and presumed MBT 2 and 4 port and stb in ballast mode. Upon the command dive,dive,dive! the vents for MBT 5, 3, 4,2 and residue venting 2 are opened. 2,4 and 3 vents are very quickly fully opened by a swift pull down of the ventlevers in the control room. 5 and residueventing 2 take a bit longer time may be 4-5 seconds as they have to be turned open. After some 5-6 Seconds the vent for MBT 1 is opened taking again some 4-5 Seconds. The boat should now have a bow down angle about 5-8 degrees, ideal: tower windeflector should pass the watersurface simultaneously with the stern. This manoeuvre is carried out in order to prevent the propellers and rudder to brake the surface. However at an angle exceeding 8 degrees you get a substantial difference in pressure between forward and aft MBTs. and it gives a crazy feeling to see from the controlroom the large engines hanging "high" up in the aft end.
The crash diving is an emergency maneuvering and can be risky, thus requires a fairly high degree of training. You have several crash dive procedures, while surface cruising on diesels, having gunnery people at the bowgun, surfacing, charging batteries, exhaustblowing of tanks to mention some, they all had to be exercised.
You reading about people preparing the bridge after the diving command could be a misunderstanding. The CO. shut the top hatch soonest possible after the dive command. A possible situation could be the early war situation while carry out sinking by gunnery and a sudden air attack. I don`t know the procedure for that situation, we never had a casing gun.
Hopefully this might give you an impression and a possible time estimation of the diving time bearing in mind that a use of Q and increase of speed would improve the matter.
Tore