The bodices in this time and place were trying to keep the shoulders thrust back. The back is one third of the total bodice size, leaving two thirds for the expansive and modestly covered front and sides, which have a softened-cone look to them. They appear to cut straight or curved across the chest from armpit to armpit, hitting right at the cleavage line in the center. The structure of the back is a deep-v shape, cut straight across from shoulder to shoulder, sometimes curved down to show some shoulderblade.
The shoulder straps do not appear to be part of the inner bodice, only of gown and lining I believe. To me they appear to have been fit from the back to the front. On the back they are in line with the pattern, but on the front they wrap softly around the upper arm into the armpit. It looks like this will be where my final fittings for the gown will be. I have read of issues with cording the shoulder straps from a few ladies, but as I recall they were running the cording up the front. I may try running my line of cording so it continues up onto my shoulder in the back of the bodice.
At Value Village I found a cheap tablecloth of 100% linen in a plain weave, which should be perfect for the 'between camica and cords' layer. The linen I found was originally a very vivid 'international orange' as I've heard it called before. I used two batches of Rit Colour Remover to suck some of the colour out as I don't want to end up with a stained camica. The colour remover worked, the linen is a faded orange colour, which is fine as I intend on having the gown be one of the prototypical Florentine colours - either red, orange, or mustard.
Hemp cord is expensive and froufrou these days, but I've found a simple jute for cheap. Jute and hemp's properties are quite similar, so I believe the jute will do quite nicely for the cording, other than the strong sizing smell that should come out in the wash.
Between the cording and gown layers I intend on using a nice tight cotton, which I have plenty of. I chose the cotton over a stretchier material, as structural stretchyness of the inner bodice might make the gown bodice wrinkle oddly when not being worn. The same cotton will be used to line the gown's skirt. The cotton I have is white, I'm considering whether to dye it to match the wool's final colour.
The gown will be in a plain weave wool I've found, I'm leaning toward a orangey or mustardy colour - the mustard is often seen on servants (I'm not going for a fine lady's dress, my wool is too simple for it).
Jen Thompson's Everything you ever wanted to know about boning with hemp cord, but were afraid to ask!
Jen Thompson's Florentine dress diary bodice construction
jennylafleur's 2004 Italian Renaissance bodice
newgarb's excellent Campi hemp bodice
sempstress' courtesean gown petticoat bodies
The Tudor Costume page's corded bodice
Ashariel's corded bodice for a florentine gown
Daria Montferrante's excellent writeup on bodice construction
see La Cotte Simple's 'Fitting bust-supportive gowns for the 14th and 15th centuries' in PDF