MmmmBeer,
We are not particularly high gravity brewer's but we're not sure your problem is cased by the gravity alone.
It seems to us that your basic problem is in getting your equipment settings nailed down. This take a bit of work but its not bad.
For a moment think of volume and gravity as separate issues. If you are set up to make a full wort boil, then you should only need to add water if something is not set up correctly. You must carfully account for all of your volume losses in the equipment set up. Easily done by carefully measuring volumes at each step. You need to get them set up so that you never need to add water.
The first and most common loss is in you MLT due to the false bottom/manifold/braid system that you use. You can set this by carefully measuring your run off using just water and no grains. The difference between water in and water out is your loss. Tipping is OK. Just remember that if you set it up tipping, you must always tip.
The second is also in the MLT but it is due to absorption by the grains. As above, this can be accounted for by careful measurement of your first running. If the loss above is set, then the difference between total mash volume and your first running volume is your absorption loss. It is set by dividing the loss by the number of pounds of grain in the MLT. The default is very close for most grain bills.
The third is your Evaporation loss. This is determined by the shape of you boil pot and the vigor of your boil. it can also change a little due to ambient conditions of local temp and humidity. This can be measured by just carefully boiling a carefully measured volume of water for one hour. The difference of water in minus water out is this evaporation loss. The water out measurement should be made before chilling so be careful. Any small variation due to ambient can be accounted for below.
The fourth is your Loss to Trub and Chiller. You can get a start at setting this by measuring the run offs using just water. If you chill with an IC, then you won't have a chiller loss. If you use a CF (counter flow) there can be a fair amount of wort left in the coil. Like tipping, you can try to get it out. If you set up by doing this, you must always do this. Plate chillers have a very small loss that is more difficult to recover. We use a plate and don't try as the loss is small. This only accounts for the chiller part of this loss. The trub loss must be done by trail and error. Some wort is almost always left in the boiler to prevent pulling too much junk from the bottom of the boil pot into the ferment er. This is where we make all of our fine and final adjustments. Start out with .25 or .5 gallons. As you make a batch and all other losses are set, you adjust this to a value that allows you to collect your required batch volume with clean wort.
The 4% shrinkage due chilling is usually close enough.
The last place where volume errors occur is in the sparge. If you Batch sparge. Then its just volume in should be voulme out as the grains are saturated. Remember, if you tip to make your volume, then you must tip with each running of your sparge. We usually set up to batch sparge using 100% of our tun volume, with equal sized batches. We always drain the tun before sparging. Fly sparging to a gravity target is more difficult as depending on you mash efficency, volume is the variable. So we don't. Fly sparging to a volume is not as complicated as your variable is gravity not volume. Many of the Grand Master Brewers Fly sparge very successfully. If you choose to fly sparge to a gravity, they will help you out.
Once you have your volumes under good control, you can attack your gravity with recipe adjustments while you work to improve your mash efficiency. Don't chase gravity/efficiency until you have volume under control or it will drive you NUTS!!
To prevent having an unsatisfactory batch while working on you efficiency, you have the two basic alternatives if you must hit a gravity target. If your pre boil gravity is low, you can boil longer and accept a smaller batch. This requires measurement during the boil to re-plan your hop additions. Or you can add extract to correct the gravity up to target. If your gravity will be high - bottle it and send it to us! Seriously, if your gravity is going to be high, you can add water to dilute it and make a larger batch than planned, or separate some to boil for yeast starters.
Wow, we said allot. Sorry for being so wordy. We hope this helps
Preston