Show me someone who's never had spokes come out too long or too short, and I'll show you someone who hasn't built many wheels. Sorry it happened on your first wheels, but even if you do everything right it might happen again.

It's important that you use spoke calculators carefully, and make sure all the data you enter is correct. The direction to round will vary with the calculator, because some give you length to near the top of the nipple head, and some toward the bottom. It's good to stay with the same calculator because you'll get used to it's bias, and can factor it in.

Also be careful about data entered, for example rim diameters (ERD). I prefer to measure them myself, finding some mfrs. specs to be off. Mixing brands of spokes and nipples can also create surprises. Most spokes and nipples of the same brand will allow the nipple to thread until the spoke is about 2mm beyond the top, giving you some margin for error.

At this stage, I'd recheck all your data and recheck the calculation. Then I'd measure everything and see if you can discover where you went wrong. If everything is consistent, then make a note of how far off you came out, and factor that in next time. BTW- spokes are measured from the inside of the elbow to the end, not from the center of the bend. It's a difference of 1mm, but when you're at the edge of tolerance a millimeter is as good as a mile.