What are you using to calculate volumes and gravity? Any software/calculator you use will take a few batches to dial in your system. I use BeerSmith (and recommend it to everyone). Other calculators are good but don't have the same features as BeerSmith. What is nice about BS is you save your equipment profile so once you dial it in, you enter your recipe you found online and BeerSmith with adjust it to your system. So if the recipe is a 5.5 gallon batch with OG of 1.054 at 78% efficiency but you do 5 gallon batches and your efficiency is only 67%, BeerSmith will adjust that recipe to your 5 gallon batches and 67% efficiency. You will be using a little more grains and hops than the 78% recipe but it will come out close to as what the recipe is as possible.
You also need to take very very good notes when brewing. Take down all volumes and all gravity readings. This will help you dial in your system. You need your grain absorption (depending on if you squeeze the grains, let them drip or drain your tun and leave whatever water the grains absorbed in there. You need boil off which is easy. Put in a certain amount of water, boil for an hour and measure the difference. You can boil for 30 minutes and multiple by 2. Then you need trub left in the kettle. So basically the volume you have after boiling minus what you put into the fermentor. I'd say typically .5 gallons. After a few batches or measuring all those, you will getting your volumes and gravity pretty damn close to estimated.