Step by step instructions for figuring out # to cast on for long sleeves - (also see below for lengthening body)
Measure from underarm to wrist (or as long as you want your sleeve.)
Multiply inches needed by # of stitches per inch (this would be post blocking gauge).
Round up or down so that it equals a multiple of 12 +6. This will be your NEW cast-on number for Chain A.
To figure out the back number (since the number of stitches across the back stay the same even if sleeve length changes), take the original sleeve cast-on number for your size (or the Original Chain A) and subtract it from the original back+sleeve cast on number (the original Chain B). This gives you your back number.
Then add your NEW Chain A (sleeve number) to it to get the new back + sleeve cast-on number (this is your NEW Chain B). When you bind off for the arm seam, the new sleeve number is the number that will need to be bound off. Make sure that you have the correct number on the needles after the bind off.

I wanted long sleeves so I figured out that I would need to cast on 90 stitches for each sleeve (needs to be a multiple of 12 + 6) …So doing it this way, gave me a cast on number of 276 (90 for Chain A and 186 for Chain B) for the size small and will yield sleeves that are approx. 17.6 inches long when measured from the armpit to the wrist. To keep the arms from being too loose at the wrist I did one fewer repeat (11 repeats total) of Chart A. NOTE: If you add length to the arms, it could mean that your lace charts won’t line up before and after the neck/back ribbing. You may need to adjust your lace over one way or another by half a chart repeat. Just know this going in so you can adjust for it rather than having to tink, like I did. For me, this meant that I started the charted pattern after the ribbing section in the middle of chart A, so with the second of the two purl stitches running down the middle of the chart. The next charts were shifted in the same manner, as well. Depending on the number of stitches added for the arms, you should either be able to start the chart in the middle (with the second of the two purled stitches) or work the chart as written. If you have to shift the chart more than that, it might be that you bound off incorrectly (make sure you bind off the side that was your provisional cast-on for the neck.)

To add a bit of length to the body, I completed one extra repeat of the 8 back/neck ribbing rows (16 total), one extra repeat of Chart A after the back ribbing and two extra repeats of Chart E (well, sort of Chart E. I kept increasing as written.)

I bound off using a plain knit bind off in pattern on the 6 edge stitches and across the back and with Jeny’s Surprisingly stretchy bind off on each of the side panels.