There is a handy little formula for predicting the OG of first runnings

oP (first runnings) = Yield Potential (in percent) / L:G + Yield Potential (in percent)

To keep it simple say we were doing a single malt mash with malt having a Coarse Grind as is Potential of 76% (Yield Potential)

We choose to mash in at 5:1

oP=0.76/5+0.76= 0.76/5.76= 0.1319 or 13.2oP giving 1.0528 if you want the density in SG terms.

Clearly you need to manipulate the numbers a bit more if you want to express the outcome as efficiency, but for an in-sparged mash (like maxi BIAB).

Say we mashed 5kg of malt in 25L of Liquor (5:1)

We have to figure that the malt will retain around 0.8L/kg (depending on fineness of grind, drain time, how hard you squeeze...)

There would be (5*0.8) 4L of liquor left in the malt, so at the start of the boil there would be 21L with an of OG of 1.0528, or 21*1.0528=22.11kg of wort at 13.2oP, from that we know we have obtained 22.11*13.2%= 2.92kg of extract.

the 5 kg of malt at 76% means 3.8kg was available.

Efficiency is 2.92/3.8*100 = 76.84%

If your half handy with excel, you could whip up a spreadsheet that lets you goal seek most any of the variables you want.

Mark

PS from Handbook of Basic Brewing Calculations Page 19, if you are going to screw around with brewmaths - its a very handy book