Why do you need a shirt? If you measure the 5 3/4" from the tip of the thumb, the shirt doesn't matter. It only matters if you want to base it off the shirt.then returned to stock. But this may not be the case. First off, your shirts should be the correct length before altering the jacket, especially if you are like many people and have one arm longer than the other. Be sure to wear a shirt which has been altered to your satisfaction when measuring for your jacket. He will measure from the tip of your thumb to the end of your jacket, allowing for the proper 1/2" of cuff to show with your arms at your sides. He or she can make both sides of the jacket show the required 1/2" of cuff. The only time I have run into this is when I bought a jacket that already had working buttons installed. The tailor had to make one side of the sleeve a little long to compensate for the previously installed buttons. Keep in mind when you are having a department store do alterations, that those tailors have almost 100% of their customers wear their sleeves too long, so they may balk at adjusting your sleeve length correctly.