Am starting the boil on a BIAB "5 gallon" kolsch.

10.3 LB Weyermann Pils

.5 LB Wey Vienna

-doughed with 5.4 gal to reach mash temp of 149, 75 minute mash

-raised to mash out of 170 for 10 min

-2 gal of 168* water for a single batch "sparge" (I remove the bag from my kettle and place in an empty fermenter with the sparge water for 10 min, and teabag it a bit :-)....took a brix measurement, got about 1.75 gallons @ 1.018/6 brix from this step)

-I squeeze my brew bag like it owes me money after the sparge, collected 7.25 gallons at 12 brix/1.048 pre-boil, so 348 total gravity points

by my calc, this is somewhere between 87 and 89% efficiency (10.8lb total grist, times maximum yield of 36-37 points per gallon per pound, or 399 gravity points....divide 348/399 = 87%).

I am considering adding some water volume, but is this possible? Help!