So, is the theory of making a more 'concentrated' wort from grain and then 'diluting' in the FV a reasonable sound one?
Glad to hear you are enjoying it all Gyro
I personally think when you want to get more out of your kettle than a full-volume mash will allow, the rule to follow is to add as much water as soon as you can and allow as much of this water to touch the grain before the boil.
You have allowed as much water as possible to contact the grain from what I've seen. I think though that after you did your sparge, it would be much better to top up your kettle before the boil as much as you can. (This increases hop utilisation and also means you have a more sterilised wort before going to the next step).
Doing the above also allows hop additions to be calculated more accurately assuming you don't top up during the actual boil.
From memory, there is a general guideline that you don't really want to dilute what you get in the fermentor by more than 30%. In other words, if you end up with 10 L in the fermentor, you really shouldn't be adding more than another 3 L. This rule makes sense to me but rules are made to be broken I suppose.
Maybe before your next brew, let's try and plan it out a bit and see what happens.
I'm working on a "temporary" calculator at the moment (while we are waiting for the BIABacus) that allows for top ups throughout the brew day and might even have it ready for a test run before your next brew.