BIAB in an urn has one very annoying aspect, which you are discovering for yourself. If you start with a small grain bill (say 4kg of pale malt and a kilo of rice) in 32L of liquor then when you hoist the bag - for those who haven't done it, rice seems to 'disappear' from the bag - the resulting spent grain 'ball' is quite light and the level of wort is "x"
Next brew you use 6kg of malt, spec malts etc in 32L of liquor and when you hoist the bag the grain ball is much bigger and heavier and even after a bit of squeezing it 'steals' more liquor out of the urn than the first brew did, and the level of wort is "y"
y is usually significantly lower down in the urn than x and if you are not careful you can end up with a couple of litres or worse of wort left over, which of course has lowered the gravity way off what you planned, or you can end up with not enough wort to fill your cube / fermenter etc.
Not being much of a mathematician, rather than bending my brain with calculating initial strike liquor level, I go from the other end and (I no chill in a 23 L cube) aim for around 24L after the boil - The Birko is quite good for loss to trub, usually only about a litre. The important thing is to recognise what is the 'right' wort level PRE BOIL that is going to give you that 25L - for a 60 min boil and also a 90 min boil.
After around 120 BIAB brews I can just eye it off and know, on my equipment, but I suppose I should get off bum and do some more accurate measurements and some calcs and maybe try to get some sort of table together and post it on the BIAB threads and the new BIAB forum as well.
I'm doing a heap of brewing for the comps this year so this would be a good project. Now for practical purposes, knowing what the wort level pre boil should look like, I can tweak by either boiling an extra 15 or 20 mins (too much wort) or just a kettle or two topup through the hop sock, doesn't do any harm to wash the hop products through a couple of times.
I reckon you should fairly quickly get on top of this, bullsneck but I know it can be a bit frustrating to start off with.
