Sleeve length question

Hi all! I'm finally starting on my TYC after staring enviously at everyone's finished products here. I started with the sleeves and have a construction question pertaining to length-- since I've never done a yoke-style sweater before.

The length of the sleeve I have finished (unattached to the body) is at approx 18". Now, if I put that on my arm on it's own, how far up into my armpit should that reach? i.e., how much additional length will the sleeve gain once I start attaching it to the body? Because if I pull the sleeve up to my armpit or thereabouts, the sleeve is probably .75" above where I would like it to hit on my wrist.

Thanks in advance for the help!