Calculating dead space

My last two batches have come up short on the final volume after transfer to the carboy. Short by a gallon!!!!
I use a three tier system utilizing a pump and CFC and thought I figured in enough extra grain and water to compensate for beer left in the CFC and pump when the keggle ran dry, but obviously not.

What is a good rule of thumb for brewing 5 or 10 gallon batches?
Say if I were brewing a "5 gallon" batch but wanted 5.5 in the carboy, with my typical system, what extra percentage of grain and water do I need to add?

Anyone else face these issues.

the easiest way to determine this is to run clear water through your system in your normal brewday fashion. Then, you can isolate how much is left behind in each vessel to know how and when to compensate.

Ideally, you'd invest in a brewing software to crunch the numbers on the fly for you so you will see the calculated effects.

I am going test the system this week. Maybe my evaporation/boil off is crazy high. For the first batch I started of with 7 gallons pre-boil, boiled for 90 minutes and ended up just shy of 5 gallons in the primary. So the next batch I compensated and had 7.5 gallons pre-boil, boiled for 90 and transferred exactly 5 gallons???? Now unless I was just totaly bombed and started drinking the beer from the keggle or a beer fairy sucked it up
I figure with my system(check profile), I would only have maybe .5 gallons left in the CFC/Pump.