at altitude I've been shown examples with an almost black triangle in one part of the sky going through to light blue in others. I think you're pretty much OK for this above 24mm?
I've got a load of Cokin P stuff too which you can borrow.
again, you sure?
I take it the Cokin P and Hitech filters / holders are fairly interchangeable etc.
shot at 19mm with a cokin filter. i think you will struggle to file that off hahaha. https://www.facebook.com/photo.php?fbid=10151531347840149&l=1ca5fe6dc9
(Durham brewery)