Alright, this question is to the masses, but I had to add in Tasty in case he is paying attention. I have had a few occations where I have needed to brew up extra strength wort, then dilute back with boiled/chilled water to reach desired starting gravity PRE-FERMENTATION. This has worked great the two times I needed to fill a 60 gallon barrel in a relatively short period of time ... 1) no-boil berlinerweisse with two double strength mashes collecting 15 gallons each, then diluting down with 30 gallons of boiled and chilled water pre-ferment. 2) a flanders red recipe that was mashed and boiled at 35% higher gravity where I could brew 4 consecutive batches then dilute down to the appropriate starting gravity with the last 20 gallons of water. Both of these were pre-ferment dilutions and they worked out great.
Now I am about to brew a mild which has a low starting gravity of 1.036-1.038 and was thinking about Tasty's "imperial" dortmunder golfing beer which I believe calls for dilution post-ferment. I am intruiged by the idea of using my 10-gallon controled fermentation space to make 20 gallons of beer, but had a few concerns with doing this with a mild. At normal strength, the 1.036 wort would ferment down to 1.011. If I double everything up to brew a 1.072 wort I would need the ferment to stop at a desired 1.022 (Wyeast London ESB). <-- that seams like a bit high for expectations of final gravity readings (Mash at 154). If I end up below the F.G. of 1.022, then the dilution may be good for bringing down the alcohol levels but will also bring down the final gravity too much ... resulting in a watery-thin beer? Should I use Alcohol % or FG to determine dilution rates?
Assuming I hit the desired FG of 1.022, I would just boil 10 gallons of water and put 2.5 gallons into 4 corny kegs then top off with 2.5 gallons of fermented wort in each keg. Sound reasonable?
Thanks for any input, thoughts, personal experience on this (Tasty's) post-ferment dilution technique?