Most of the runnings drain quickly from the bag but there's always a little more that's been hard to capture without holding the bag until your arm cramps or you get around to installing an engine hoist in your living room...until now.
Using a 2.5G Rubber-Maid bucket wedged in the sink...my grain bag is 'suspended' at an elevated angle...which allows the last precious drops of wort to be drained and captured easily.


The 5G grain-bag is held in place by folding the top under the bucket.
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Note the thin wooden skewer on the counter with the black mark on it in the second brew-day image. When the skewer is inserted into the boil kettle...the black line marks the level of my 4G pre-boil volume.
There's no need to put 4Gs of water in the pot to figure out the 4G height. All you need is a ruler...a calculator...some basic algebra...and a home-brew.
Volume of your pot equals Pi times the squared-radius of the pot multiplied by its height.
V = Pi * r^2 * h
Pi = 22 / 7 = 3.14
Cubic inches per gallon: 231
Radius = 1/2 diameter of kettle (my diameter = 11.75")
V(gallons) = (Pi * radius^2 * h) / 231
Plug'n
chug for h, the height (in inches) for 4 gallons
4 = 3.14 * 5.875^2 * h / 231
4 * 231 = 3.14 * 5.875^2 * h
924 = 108.4 * h
h = 924 / 108.3
h = 8.5"
Mark your stick 8.5" from the bottom to indicate the 4G level.
See...despite what you thought about it in HS...algebra really does come in handy in 'real life.'