I do half and half, or maybe 60/40. Lots of people do bigger mash and smaller sparge.Roughly how much water would you sparge with?
Even a small volume sparge will increase your overall efficiency by rinsing out some sugars that would have been left in the grain bag no matter how much you squeeze. You can (and should) adjust for losses due to grain absorption (maybe one liter per kilogram of grain, maybe less) and boil off (depends on how vigorously you boil and whether that burco is short and wide or tall and narrow; could be as little as two or three liters per hour or as much as nine or ten). Measure and record all of your volumes. After a few batches you'll know what your own rates are.Seems easier to do close full volume and just squeeze the bag into the boiler. Probably wont end up with full 20 liters as I'd have to account for water displacement