Tutorial - altering a pattern - Gathers

Hi! I'm gradually getting movement back after my ankle break. I still haven't willed myself to write up my chalk bag tutorial, but I have started on what will hopefully be my outfit for my sister's wedding in August.
My statring point is this picture of my Grandad and Grandma getting married. This was June 1946, a year after the end of WW2. Rationing was still very much in place in the UK. Looking at the picture, I am certain that Grandma made this outfit from a large man's suit. If you look at the little bubble at centre front I think that is were she turned a man's notched collar into something like a ladies' shawl collar. And I am sure the pleats down the skirt used to be pleats down trousers! Grandad was still in the airforce so he got to wear his uniform.

My mum bought me some 50s vintage buttons for Christmas so I decided that rather than make something literal and 40s, I would take the gathering detail from Grandma's outfit and put it on something more 50s. I had enrolled onto the Sew Retro Starlet Jacket tutorial, where the brilliant Gretchen Hirsch takes you through the tailoring of a lovely 50s jacket. The jacket in the course is exactly the shape I want, so all I had to do was to add the gathering details.
The first step is to trace the pattern piece you want to adapt. There are a few reasons for doing this. I want to make the jacket without gathers one day, also the original pattern piece is very useful for comparing the gathered peice to (and I might use it to make an interlining to control the gathers. Also you might be as daft as me and start doing this on the side back, not front!!! Mark all notches and grainlines. I also marked where the waist is.

The next decision is where to put the gathered section. This is a design decision and there's no science to it. I didn't want to interfere with the princess seam (and I didn't want the gathers over the bust anyway) and I didn't want them to extend down to the hips. I drew in the grainline and then drew parallel lines perpendicular to the grainline at 2cm intervals.

I made some notes of the distances between points so I would know if I had the right amount of gathering on the finished piece.

Then I cut up the pattern along the horizontal lines

I drew a grainline and more horizontal lines 2 cm apart on a bit of parcel paper. I drew the lines 2cm apart as 1:1 seemed like the right amount of gather. If you wanted more gather you would alternate between a 2cm gap (where the cut bit of pattern goes) and a bigger gap. For less gathering alternate between 2 cm and a smaller gap. Arrange the cut pieces being very careful to match grainlines.

I marked the edges of the pattern at each 2cm line. They won't make a smooth line yet.

I smoothed out the lines with a pencil and a French curve, then cut out the pattern so I could make a toile and make sure the gathers are how I want them.
As I've not made this pattern before I made a full toile to check the fit, but if you are doing this to a pattern you've made thousands of times before then you could just make one front to check.

I'm pretty happy with the gathers, and very happy with the fit. I might control the gathers a bit more when I make it out of the fashion fabric, but I think the overall effect is pretty flattering.