Does that water (Yeast Starter 3L, 16L and 12L not add up to 32L)? What am I missing here? Do I split the sparge from the 16L? Use 4L for grains?
I assuming grains will drop water temp from 79 to 66 yes?
By runoff, do you mean removing grain bag, and sparging into same wort in kettle? I assume so.
How fine to mill grains? LHBS will mill them for me, or I can do it at store. What instructions? Pretty sure they will have mill preset.
Last question - Maybe!!!!
I saw a guy on youtube - a crasy aussie, using his stove oven - preset to 150 C to keep his wort temps constatn. What else would you advise? Can I keep heat on burner? Think I might have to use a stove top.
From the yeast starter, use only the yeast sediment at the bottom and throw away the liquid. This is commonly done for lager beers. It will only be a few mL of yeast sediment and that is okay. Most of the 3L is then gone, not used. You can drink some if you're interested to see how it tastes.
Mash with the 16L, and sparge with the 12L. I wrote the recipe for mashing in a cooler, so some of the terms about runoff etc. might not apply, but I think you understand.
The grains will permanently soak up roughly 835 mL/kg of grain unless you squeeze the bag hard. Personally I don't squeeze, I've assumed a traditional mash and sparge in the instructions above. If you squeeze hard the way many BIAB brewers do, then you'll need less water. I'm not sure how much exactly, maybe the permanent loss goes down to 200 mL/kg but that is only a guess as I don't brew that way. My analysis assumes no squeezing, hence the significant volume loss you detected here.
When it's time to sparge, you can do as I do and pour your hot 16L sparge water into a cooler or big bucket, then dunk the bag into it, stir well for a couple of minutes, then pull the bag and throw away the spent grains, and combine the sparge liquid back into the kettle and brew as normal from there. To get more fancy, you can strain all the liquid again through a fine mesh bag to remove more of the tiny grain bits. It's optional but I do it. Most BIAB brewers don't.
Mill the grains very fine. There should be a fairly high percentage of flour after milling, I would say 15% flour or so, and every kernel should be broken up into at least 6 or 7 bits. At your shop you should request to mill them through at least twice because shop owners usually set the gap too wide and it results in low efficiency. Longer term you'll just want to get your own mill and not worry about what the shop does with their gap.
I have never used an oven to hold mash temperature, and frankly I think it's unnecessary. If you are concerned about losing temperature during the mash, don't be. Most of the conversion is done in the first 20 minutes or so anyway, and you can always plan to start the mash a little warm about 67 C and just let it fall to 63 C by the end of the mash, and it won't hurt anything at all. If it gets any hotter or colder than those temperatures, I would add a little heat on the stove top or a little cold water (we're talking just a cup at a time) to bring it down if needed, but anywhere in the 63-67 C range is like Goldilocks, perfect.
P.S. Yup I got the Jefferson's reference. Finally get a piece of the piiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiie.