The process to make a sweater that fits is easier than most knitters might think. Starting with the measurements from a finished, favourite-fitting garment, the knitter is armed with Tool Number One to knit the correct size sweater.
Please note the word FINISHED. That is the measurement you are looking for. Some patterns give more than one chest measurement. The pattern might give a To Fit Chest Size measurement as well as a FINISHED chest measurement. You measured a FINISHED sweater, so you are looking for the FINISHED chest size measurement to start your work.
The difference between the To Fit Chest Size... measurement and the FINISHED chest measurement is the amount of ease the pattern is suggesting for the sweater.
I have a funny story about ease. When teaching my 'fit' class one year, two friends came together to take the class. These ladies were, as my Mother would have said, Mutt and Jeff. That is, one quite chubby and the other quite thin. They provided the perfect example of why it is important to measure a sweater and not yourself.
Both ladies had brought red sweaters to class to be measured. Miss Chubby's favourite-fitting, red sweater measured a figure-hugging 44 inches. Showed off her curves. Miss Thin's red sweater also measured 44 inches. It fit like a Mu Mu. Remember these were their favourite-fits. These two ladies, with vastly different sizes and figures wore the same size! That is how they liked to dress. If they had chosen to knit the To Fit Chest Size in their pattern, they both would have been disappointed in the fit of their sweater. You measure a FINISHED sweater and you chose the FINISHED chest size from the pattern that is closest to your sweater measurement. Digression complete.
If you want to knit a 40 inch sweater and the pattern has a 40" FINISHED chest size, you are in luck. Simply knit that size. But if the FINISHED chest sizes in the pattern run differently - say 30,34,38,42,46,50" to knit a 40 inch chest, then you need to go back to the calculator.
My method is to look at the difference in cast-on numbers for the two sizes on either side of my 40 inches. What is the difference between those cast on numbers?
For example if size 38 calls for a cast on of 95 and size 42 calls for a cast on of 105, the difference is ten stitches. Since 40 is half way between 38 and 42, I can easily calculate the half way mark between 95 and 105 and cast on 100 stitches. AND - carry on with the half-way numbers all the way through the pattern.
For example, if the pattern tells you to cast off 4 stitches at the underarm for size 38 but 6 stitches for size 42 size 40 would be a cast off of 5 stitches. Half way.
And to paraphrase Elizabeth Zimmermann, - One stitch either way won't make or break a sweater.
Length, of course is a different. Tomorrow's story.