Ike,
some use a lens without dioptre although having not much headroom from infinity with a certain lens/dome (not this case). Must be a reason why the rule was born by some manufactures (didn't say Ikelite).
I did not comment if a dioptre with sea&sea/12-24 is required. just which would eliminate the reduced focus ability (origin question).
"A lens that can focus to under twelve inches should not need a diopter addition".
depends on close focus "desires" (origin question) and not valid for every combination.
