Welcome to the forum.
After reading your post, I did some calculating.
I had the data for a 1955 strat pickup wound with 42HV.
So I duplicated that data for a 7 string.
I based the bobbin length on .402 between strings, and .187 diameter magnets.
I would try the same amount of turns for a 7 string as for the 6 string.
Here's what I came up with.
You could also come up with something else, using 42 Single wire.
BTW what pickup position are you winding for?
Neck, middle, bridge or all?
Here's the attachments I made, and data I came up with!