I'd better get this down before my brain melts, and I come back tomorrow to look at the print out of the first page of the script analysis questions which is now absolutley covered in calculations, cross outs and a lot of "x -> w =" comments.

Here's a copy of the blow up of part of the script of the Bedford Hours from Stan Knight's Historical Scripts book

Thankfully, Stan Knight says that his blowup is 150% of the original's size. Somewhere I needed a clue of how big the original actually is. You never know by how much the reproductions have been re-sized just by looking, and this told me.
So I needed to multiply everything I measured in the Stan Knight blow up by 0.667.

Pen width (measuring on a wide part of an O) = 3 . So, if it's 150% the size of the original I need to multiply it by 100/150 or 0.667 = 2 mm.
That is the pen width used in the Bedford Hours.

I measured the x-height - not between the lines drawn inbetween which the text is floating (I'll do that later) but the height of a,c,e,on,m etc. (normal 'x-height). It came to 11 mm on this reproduction.
Again, multiplying by 2/3, I got 7.3 mm, which is what the x-height should be on the original
So, x-height at 7.3 mm, with pen width at 2 mm, that gives up approximately 4 pen widths for the x-height.

Because I'm a masochist, and to check these figures, I checked against 3 other images I had.

Codices Illustres image, page 187
Pen width on this reproduction measured 2 mm - it's the size of the original image!
X-height measured either 8 mm or 9 mm, depending on where I measured in the page.

Nice - it agrees with my calculation

Brown/Lovett Historical Source Book for Scribes
Pen width measured 1 mm.
If the pen width on the original is 1 mm, I need to multiply everything by 2 to get the 'real' measurements from this image.
X-height measured 4mm. Double that to the original's size to get 8 mm and 4 pen widths in the x-height.
Again, it agrees!

same book, blow up of some of the script (easier to measure)
Pen width = 3 mm, so multiply by 2/3, same as the Stan Knight script to 're-size' the image back to the original.
X-height measured 16 mm, two thirds of which is 9 mm.

So I'm getting a variance between 8 and 9 mm for the x-height over 3 different pages of the Bedford Hours (remembering the second Brown/Lovett image I measured is a blow up of the first).

I'll go with 2 mm pen width, and 4 pen widths (8 mm) x-height
Because the images are pretty small, it's so easy to debate between measuring 1 mm or 1.25 etc. It varies depending on which particular letter you measure (I got O's with slightly different widths in the strokes in the same image). By the time you multiply it by a factor to re-size it you've magnified any error. So I'm not that worried that everything doesn't match exactly.

Next is to look at ascender and descender heights.
The column questions before the pen width stuff on the list were easy- there aren't any. Ditto line dividers and hyphens. The margins question - don't ask. I got all sorts of different measurements for the space between the inner edge of the illumination border and the start of the text, left, right, top, bottom, and currently waiting for some help in interpreting it.
I have a headache.