Read this article by Threads Magazine to learn how to make your sleeves (and anything else) fit by using the measurements of a well fitting sleeve on one of your garments to use as your starting point.
This is hands-down the best sleeve advice one will ever need (this is applicable to all garments/accessories).
I have learned over time that the best way to fit your garments is to refer to your RTW ones for reference points. In addition to fitting, you will also learn about construction techniques from these garments.
I found this article to be one of the most informative and helpful ones I've ever read.
You'll learn how knowing the measurements of what fits you is even more significant than knowing your body measurements!
To help you get the best fit on all clothing, spend a few hours trying on garments that you like the fit of and start measuring. Draw a small basic sketch and write in widths and lengths at significant points.
Use this information when it's time to make pattern alterations on similar garments and you will have far less disappointments ~that I promise you!
Here's an excerpt from Peggy Sager's brilliant article...
"Don't measure yourself, measure what fits"
I've met hundreds of women who can tell me their measurements, but don't know the measurements of garments that fit them the way they like. I propose the following radical but practical notion: It's more useful for a sewer to know what works for her than to know what her own measurements are. (Sleeves are not the only garment details for which this idea is useful.) Once you've located your Holy Grail set-in sleeve or sleeves (a jacket sleeve and a blouse sleeve cover the set-in territory for me), all you need to do is measure it carefully in the relevant areas (including the garment's upper back, which controls the freedom of movement), then make sure that all your future garments with similar sleeves have the same measurements."
You'll find diagrams and more info at Threads Mag.