I brew using Grainfather. Our og is always very close But volumes fluctuate from time to time. Can I just add water to the cooled wort to bring levels to 5 gallons. Thanks
Yes you can always add water, but it should be boiled and de-chlorinated, so that you don't bring in tramp microbes or off flavors. Diluting will lower your OG proportional to the amount of water added.
If hitting all the numbers is important to you, then you should work to figure out why your volumes are fluctuating, and then try to control them better, as this will bring more consistency and predictability to your brewing. It's also fine to not worry about hitting all the numbers. This is a hobby after all, so do whatever gives you more enjoyment.