Does topping off affect the % abv calculation?

I'm almost a complete brewing noob and will soon be racking my first attempt at mead. Somehow I've ended up quite a bit shy of the gallon I was going for and will need to add close to 4 cups to bring it to where it needs to be. Won't this change the gravity through a method other than fermentation? And if the % abv is calculated with the difference between OG and FG won't the result then be off from what it actually is? I can see it not making a significant difference in a larger batch but considering that the addition will make up close to 25% of the total I'm thinking it could be pretty far off.

I'm planning to make a honey/water mixture to top with. Will making this mixture to the OG of my mead keep the % abv results fairly accurate?

Yes
Yes
Yes, good idea.

David is not wasting words today but he is correct of course.

Welcome to HBT!

Thanks for the info (and the welcome)! I thought that would be the case I just couldn't find anywhere it was explicitly stated. Is there a way to account for post-OG additions and still get an accurate abv? I'm planning on doing a blueberry melomel for my next batch and some of the recipes I've looked at call for adding fruit or extract in secondary. Won't this cause the same problem?

Azurescens, welcome to HBT. There is a way to account for your additions and easily calculate your changes. Read throught the Mead Sticky at the top of the mead page, most notably the honey spreadsheet Hightest has posted. Using the spreadsheet you can input your current quantities and make the additions. It should give you a rough apmoximation. Hightest's Mead Sticky is a wealth of information.

I read through the sticky but apparently not thoroughly enough. Somehow I completely missed the spreadsheet. I'm gonna read it a couple more times before I ask anymore stupid questions.

Thanks again!

No such thing as a stupid question!

One thing I've learned to is to make just a little bit bigger batch, and then use a sanitized wine bottle (with a stopper and airlock) for the "extra". I then use that to top off my racking losses. It means I can minimize my headspace and not water down my mead/cider/wine.

Home Distillation of Alcohol (Homemade Alcohol to Drink)

Here is another place that shows how dilution works. 3/4 of the way down the page, and just input #s into the "down to ___%" to see how dilution affects your mead.

Of course when you working with just a gallon, a little dilution causes a lot of change.

Thanks once again for the even more info! It just keeps coming!

Making extra for topping up sounds like a good idea. I'll be sure to do that next time. I'll probably do at least a 3 gallon batch as well.