One of the biggest frustrations with knitting garments in a superwash yarn or in a lace pattern, is not ending up with the proper length garment, armhole or sleeve. Since superwash yarns grow during blocking and lace patterns expand and open up, the length as you're knitting is not the finished length.
Pattern instructions refer to the finished, blocked lengths: ...Work until armhole measures 8"/20 cm
...Work even until sleeve measures 18"/45 cm or to desired length
...Work even until body measures 12"/30 cm
If your knitting is likely to expand, and if you work to the measurements listed in the pattern, it will be too long. Don't write off superwash yarns or knitting sweaters. You can combine both and achieve an excellent fit. Learn two methods of getting the perfect length for your next sweater by reading below or in my YouTube video: https://youtu.be/18rCAobG3_M
The first method requires a good gauge swatch. Make sure your swatch measures at least 4"/10cm or longer. Wet block it, pin it, let it dry, remove the pins, shake it out, and let it rest. Using a ruler, measure the number of rows over the entire stitch pattern then divide to get the number of rows per inch. Do not include the cast on or bind off edge. If you have a drapey fabric, hold it up and measure it while it's hanging. Your garment is going to hang on your body after all, not lie flat on a table. If your fabric doesn't drape, and if it's easier for you to accurately measure while it's flat, then do so.
Do not round this number. If you measure 32 rows over 4"/10cm, or 8 rows per inch/2.5cm, that's great, but if you measure 31 rows, then use the actual 7.75 rows per inch/2.5cm.
Next, convert your pattern from inches/cm to rows. We'll use 8 rows per inch/cm for these:
...Work until armhole measures 8"/20 cm FOR YOU, this means 8" x 8 rows = 64 rows
...Work even until sleeve measures 18"/45 cm FOR YOU, this means 144 rows
...Work even until body measures 12"/30 cm FOR YOU, this means 96 rows
If you have a repeating stitch pattern, like a chart, take this one step further and convert rows to repeats. If your chart has 16 rows, take the 64 armhole roles divided by 16 chart rows and you'll need to work your chart 4 times to reach 64 rows or 8".
The second method is used when you don't have a good gauge swatch or when you don't trust your row gauge. Your gauge swatch may be awesome, but if you find that when you hold up your knitting while still on the needles that it's drooping down more than you thought, you'll end up with a different row gauge than your small, flat swatch revealed. This is because the weight of the garment is pulling down the upper portion of the garment and you'll have fewer rows towards the top, the area that is supporting the weight, than you will on the bottom or when it is lying flat. This is where armholes and neck openings can become deeper than intended.
In absence of an accurate row gauge, the best way to proceed is to stop knitting a couple of inches before the pattern says, place your live stitches on waste yarn and wet block it. If you're working with superwash yarns, your garment may expand enormously. Don't stretch it out and let it dry like this. Refer to the schematic and reshape it into measurements that match the widths listed. Since you can't measure to the schematic lengths yet, look at the fabric and if it looks stretched, place your hand underneath it and carefully bunch it a little until it's resting on the blocking mats in a very relaxed state. Once it's reshaped, pin it, let it dry, shake it out, let it rest, then measure again. (If it's a drapey fabric, measure while holding up instead of lying flat.)
Now you have an accurate measurement of what you've knit so far. If your armhole now measures 6.5"/16cm, and you need to work to 8"/20cm, you will need to work an additional 1.5"/4cm or 12 more rows (based on 8 rows/inch). Place your live stitches back on the needles and continue knitting the number of rows you need.
Use the method you like best. Both ways will give you the confidence that you are working to the exact length regardless of how short it may look during knitting.
Elizabeth Zimmerman once said that if you like knitting in the round, then knit in the round but don't do it because you can't purl at the same tension. Don't compensate for poor skills or lack of knowledge. If your lace doesn't fit, don't limit yourself to stockinette. If you've knit unsuccessful garments, don't limit yourself to accessories. If you've ended up with gigantic superwash knits that don't fit, don't limit yourself to non-superwash wool. You can have the best of both and knit what you want. It's your knitting - make it magnificent!