Sleeves can be a personal thing when it comes to fit, circumference and length. It can also vary depending on your row gauge. To get from the start number of stitches to the end count of number of stitches you need to do a certain number of decrease rows with 2 decreases on each of those rows. These decreases are usually spaced out evenly between the beginning of the sleeves and the desired length you'd like. The pattern says the decreases and the length are approximate, not exact, so this is where your knowledge of your preferred fit comes in.
When you know what length you would like for your sleeve minus the cuff length you can then divide the number of inches by the number of decrease rows needed. If it turns out that the sleeve is too loose or too tight you can adjust the number of decreases to make the sleeves work for you.
You do not need to end with the stitch count the pattern calls for just one that fits how you would like it to fit. It takes a while to figure out what works best for the fit you like on sleeves and every sweater is different. You have the skills to do it though. Trying it on as you go will help with learning what you like.
Here is a link to one of Marie's blog posts about figuring out the length of a 3/4 length sleeve: https://www.oliveknits.com/34-sleeves-for-everyone/