There's no major theoretical reason why you can't measure it at mash-out. mally can give us more accurate theoretical numbers but as a rough guide, water (and therefore, we assume wort) swells by 2% from ambient to mash temp and then another 2% from mash to boiling point. i.e. 100 L becomes 102 L then 104 L. So, multiply your mash volume by 1.4/1.02* (you can do the calc in the VIB field of the BIABacus) and that's all the maths you need to do but...
There are some practical considerations to consider. The first major one is that it's quite common practice to pull the bag after mashing out and then leave it to drain further as the wort is brought to the boil. On an average brew (around 19 L VIP of 5% ABV beer), that practice will give you a few more litres into the boil. Some brewers doing a high gravity brew in a small kettle may make a decision to sparge and, others, may be topping up their kettle before the boil.
If you do take your volume reading at mash-out, make the volume adjustment, and then, before the boil, add any runnings you collected or water you added. If you take your volume at the beginning of the boil, when the wort starts to boil, turn the flame-off and then take your reading, as best as you can.
So, any 'kettle' efficiency reading can be taken, at any point, as long as the volume and gravity measurements are taken at the same time, the volume is temp corrected* and, that time is after all runnings from the mash have been lautered (drained).
Remember that the purpose of this first check on efficiency is only to act as a "reverse double-check"; you never act on that first check. In other words, if your ambient wort efficiency check came out low (and you double-checked that), and your into kettle efficiency reading was also low, you will know with a good degree of certainty that something went wrong on that brew. If you only make one check, your degree of certainty goes down massively.
All these things get less important once you've done several brews and know that there are no fundamental errors occurring in your brewing process. In the beginning though, the extra checks really help to nail a problem fast.
PP
* For the actual efficiency calcs, all volumes are actually temperature corrected back to ambient but The BIABacus does this automatically. In other words, if you take a VIB of VFO reading, you type in what you actually measured because the BIABacus assumes you took those measurements at boiling point.
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