Maybe a stupid question....
 

....but my brain isn't working full time today. I'll apologize in advance for the question...

But my husband wants spaghetti tonight, and we don't have enough actual spaghetti noodles to go around. I'm going to use some rotini we've also got in the cabinets for myself. The package says 1 serving is 3/4 cups DRY...so if I cook only 3/4 cup of the rotini, I could eat it ALL, as it's one serving?

Yes, assuming you've looked at the calories/carbs and agree with what they call a "serving." Otherwise, adjust from there.