I had a look at the spacing between verticals on some Bedford Hours pages. In real mss size terms, there is a variance of 0.5 mm, given a 2 mm pen width.

I actually noticed that there was variance on real mss pages (as opposed to the Gods with Snow quote in Drogin, which is 'perfectly spaced' to give the illusion of illegibility) back in
http://elmsleyrose.blogspot.com/2006/09/straight-lines-and-diamonds.html
and the two posts before that. So I already knew there was a tiny bit of variance. What memory?

I killed my last W.M. 0 nib yesterday (I kill nibs frequently).
I like to work with a large x-height when I'm working on a script, and consequently like to use a wide nib so the letters don't look too out of proportion - too tall and skinny.
I tried using a large speedball but it was too wide - just killing my hand with the amount of real estate necessary to cover in order to form a letter.
So I used my little Brause nib this morning (about the size of a W.M. 2) - and the spacing was really good! I'm pleased.
I'll have another look at the other nibs I've got and try and find something sized between the large Speedball and the Brause and practise with that, and see how I go.
Varying the nib is a good idea anyway - so I'm looking at doing the spacing according to the width of the nib I'm using at the time - rather than always practising with the same nib. I don't want to accidentally learn to space by a particular nib's pen width and carry on with that spacing when I change to a different pen nib width.
That was a problem I had with The Second Coming piece. I was used to using a W.M 1.5 and when I changed down to a W.M. 3 to write the piece, my writing went a bit kerfluey, because it'd been a very long time since I'd written Gothic Textura Quadrata with anything BUT a W.M. 1.5.