From "Analysing A Style" by Mistress Rowan Perigrynne
http://www.sca.org.au/scribe/articles/analysing.htm

Step 1

In order to analyse the basic design on the page (and be able to come up with a similar design yourself), you need to start with the big picture...

General layout
Is the layout horizontal (landscape) or vertical (portrait)? Or two matched pages?

Vertical, two pages (you can tell by looking at the size of the margins. Also by the way the pictures are distorted in the inner gutter)

Are there any borders?

Yes - rectangular even(ish) border around the text block, with decoration.

Are the any large graphic elements (capital letters, pictures, seals, devices, etc)?

An illuminated letter

a seal of arms (of William Catesby, added later on in the 15th century by the later owner)

4 large roundels (one at each corner of the border)

Margins
What are the relative proportions of the top, side and bottom margin? Don't forget that some manuscripts had the margins trimmed when rebound or for colour reproduction!

To make measurements, I measured the x-height of a text line shown on the image of 164 of Backhouse's Illuminated Page.

To give a x-height of 7 mm, I needed a multiplication factor of 1.625. So I need to apply that to all of my other measurements to get them to the correct size.

I did all of the measurements, did a couple of layouts, and discovered that it was easiest to work from the inside out, since I had an established size for the text block (normally you work the other way around, and calculate the size of the text block. see Edward Johnston)

I have the guidelines, of 18 lines at the correct size, which forms my text block. The text block measures 14.8 x 23.2 cm

This went into the middle and then I added the border (1.3 cm all around) and then the top and bottom margins. (top, 3.2 cm; bottom 9.1 cm;).

The bottom margin is very tall because of the amount of marginalia in it. (sprays of flowers, the shield). Visually, the empty space at the top and bottom margins look in proportion.

The right hand side of the image is curved because of the binding of the actual Bedford Psalter book, (my image is of a page on the left, f151b) giving an incorrect measurement.

I ended up choosing the width of the right side margin that I needed to fit the illumination nicely, and then adjusting the left hand margin to equal it. (and would fit the left hand marginalia nicely).

(I'm doing this as a single page, not as a double page spread, so the " the inner margin is half that of the outer margin" need not apply. They can be equal.)

One thing to consider (that might lead me to change this) is that the marginalia is very narrow on the right hand side, because the page was designed to have an inner margin on that side. The marginalia on the left hand side is much wider.

The roundels, with radius of 2.25 cm (top roundels) and 3 cm (bottom roundels) are also marked.

Good scans hey, with the measurements clear as mud. Still, it shows the text block, borders, roundels and margins. (well, part of the margins).
I didn't bother to patch the images together. The start of the repeat can be seen where the "1.3" width of the left and right hand borders is marked.

I need to add a left hand margin of 3 mm to the text block. (The right hand margins are on the guidelines)

This is the actual page, to show the layout :-

(I'm not doing a shield)