Continuing the topic of re-sizing historic patterns, here is how to re-size the back & front of a waistcoat. You can use this method for any jacket/top, even if it has more parts than just front & back.
Begin with your original scale drawing (this is a 1770 sleeved waistcoat from Cut of Men's Clothes). This is the one you keep on file & don't cut.
Don't try to copy these images, the camera angles have distorted things.
The pieces are:
Pocket (not shown)
Trace the back onto another piece of pattern paper, (this is the one you will cut).
Measure the back waist, Paul's is 21". This is the base of the neck to the natural waist in back. The actual garment measurement will depend on the cut, but this one is exactly the same, 21". Lengthen or shorten at the middle of the back, about where the ribs start. (this is not pictured here, the original pattern was right for him).
Measure across the back & multiply by 2. This is the current back width of the pattern. (I'm lazy & fold the tape measure over).
Measure your back width, (from where your arm meets the back, across the back, standing naturally).
You should add 1" to your back-width minimum.
Paul's has a 19" back width. He will need a minimum of 20" to be able to move, but no more than 21" as it will be too big for him.
Extend the back width evenly on either side.
If more is needed, continue to extend the measurements.
(Depending on the garment I may add more to the center back than the side back, you will get a feel for this if you do enough.)
Draw in the new center back (do not draw in the new side-back yet). Extend this measurement all the way down (this may get re-adjusted later, but this helps to maintain the shape of the garment).
note: always measure in the middle of the ruler, ie: 2"-3" rather than the 0"- 1" as your lines will be more accurate.
Trace out the front.
Make any length adjustments that you need.
Be careful to match the side seams in length. This may change as you make adjustments, but it's a helpful guideline. The underarm-waist measurement will be useful here.
Measure the chest on the front (minus button space) and add it to the chest measurement in back.
For an average size person, you will need 3-6" of ease depending on the garment. This one needs 1" in back & 3" in front, including the buttons... so 4" larger than the chest, total.
Make any adjustments to the chest & re-check.
Repeat for the waist. Depending on the shape of the individual, you may be adding more to the center front than to the sides.
Always add equal amounts to the side front & side back seams.
Remember to extend the skirts in proportion to the body, but maintain the angles unless the individual is heavy, then increase the angles.
Adjust the armhole, shoulder & neck accordingly.
(See the previous post).
In this illustration you can see how some of the shoulder angles have changed.
When you lower the armhole, do not go much lower than 2" for 18th century garments. If the person wants more movement, cut away from the front of the armhole rather than the underarm. Even then, only cut away what is absolutely necessary. Make sure it isn't cutting off circulation at the back of the arm.
Double & triple check all your measurements to make sure everything will fit together. Clean up your lines & measure again.
Now you can move on to the small parts.
For the pocket, trace the original onto your sized pattern (green) & increase it accordingly (red).
Maintain the angle & line placement from the original garment. If your person is much taller (Paul isn't), extend the pocket too.
Be careful that it doesn't get too big, but make it large enough for the wearer to comfortably get their hands in. If this is sized-down for a child, make the pockets "mommy" size so you/she can fish out whatever lovely things the little darling has collected.
The pocket itself should end 1/2" -1" above the hemline, or no longer than standing fingertips. This prevents the pocket from catching when you line it, and the "lean & fish" while you wear it. It should be the same width as the top of the pocket flap, but the hole should be 1" in on either side, so there's no chance it will show. If the pocket flap is curved, make a smiley face hole, if the flap is straight, make a rectangular hole.
Draw your final lines in pen or marker.
Remember to add in your grain line, button placement, clip marks, etc.
Also include the garment name, date, person it was made for, individual measurements (in case you run across someone the same size), and where the pattern came from.
Size your sleeve accordingly & re-trace the under sleeve. Make up whatever you can in the gusset, remember to keep things in proportion.
(I cut& pasted the gusset here, it's not in proportion).
The upper sleeve has the under sleeve marked in as well, but is not cut from the same pattern piece.
Add in your seam allowance, I prefer 1/2", but 5/8" works just as well.