Alexandra >>the variation that comes about when people write messily or hastily
Exactly! And regional variations, I've found, as well.
If you think it would help—you're probably way beyond my abilities, but lots of visuals is sometimes good—my favorite book on the old hands is "If I Can, You Can—Decipher Germanic Records" by Edna Bentz. She has died, but her children keep it in print. It's realllllllly low-end, in terms of printing (copy-shop style), but REALLY useful in terms of content.
Of course it's aimed at genealogists so some sections may not help you much, but the wealth of examples she reproduces is superb!