There are a few resources which anyone approaching pattern adaptation…
E is for … Ease
Ease is the amount of space between your body and the garment. ‘Negative ease’ on the other hand is where the garment is smaller than the wearer’s actual measurements.
Ease will vary depending on the style of garment you’re knitting and is something you’ll have to use your judgement for. The standard ease for modern garments is a comfortable 2″ to 4″ (5 – 10cm), but between the 1930s and 1950s, when a tighter, more fitted look was favoured, garments tended to have a negative ease of -1″ to -2″ (-2.5 to -5cm), which means the garment was knitted smaller than the wearer’s measurements.
When you approach a pattern, particularly one from the mid-20th century, one of the first things you need to decide is how much ease you’re comfortable with versus the style of the original garment. Too much ease and you will lose the fitted vintage look, whereas too much negative ease might be uncomfortable.
I tend to steer away from too much negative ease, balancing a fitted look with something I’ll be comfortable wearing. The measurements I use in my reconstructed pattern planning reflect my measurements, giving a fit that’s close to the skin with maybe up to 1/2″ (1.25cm) ease depending on the garment, but of course the decision is yours.
Don’t forget, you’ll need to cater for more ease when you’re knitting a cardigan or jacket to fit over garments you might be wearing underneath.