First the Parrot; Yes, a Parrot will make things less messy for you in that you'll have a constant general indicator of what's coming from your still but remember it is an averaged figure. There is simply no substitute for your taste buds and your nose. Having said that, if you have one you'll always use it.

Output from your still is a great variable because no two washes are the same - even if made with seemingly identical materials; Operating temperatures vary, how clean/efficient is your still on a particular run, water quality on the day, etc, etc, will all have an effect, not just on the ABV of a wash but many other factors too.

Run 1: The vol you get from a stripping run in subject to all those things up above. 1.5Lt is generally what I take from each batch but I have a well nailed down routine. What you take from 4Lt of wash depends on what's in it to start with. So the vol you can take from each 4Lt of wash is dictated by when the alcometer gets to 30% and you may well take more than I do because I'm not so concerned with volume (there's no particular reason why you can go down the 20% on the stripping run, the 30% figure is just what suits me)

Run 2: Spirit Runs; Again 1.3Lt is from my set up and I used those figs as example only. As above it's subject to all those variables. The foreshots is easy - chuck 50 ml regardless. The Heads is a matter of judgement and personal taste because that's how you know you've reached the end of the Heads - taste!

In my case I generally get 300-350ml of heads from my Airstill before getting into the Hearts. The 1.3Lt I mentioned includes Heads & Hearts.
When I was starting out I had the same questions as all noobs and what I did, in the end, was decide to treat the first 500ml, after the foreshots, as heads. Over a number of washes I collected this 500ml in 100ml batches in little jars numbered 1-5. I let the jars air overnight and the next day I smell tested and finger tasted each jar starting with number 5 (Don't swallow when sampling and rinse with water between each sample) It took very little time to find that point, the one jar, where the Heads disappeared and the hearts started. I was then able to differentiate between to two tastes. I quickly learned what the exact taste and smell to look for so now I finger test what comes directly off the still - a single drop on my well cleaned little finger straight to the nose then a second drop straight under my tongue. You'll be surprised how quickly you'll get to that point too
The maths you suggest at the bottom of your post - I wish!

As far as I can determine there is no direct mathematical correlation between the wash strength and the output. For example, if your supposition was correct then 4Lt @ 40% would become 1 Lt @160%

The ABV of your output is constantly changing with almost every drop from the still. Therefore what's in your total collection had some output at, say 83% but you stopped collecting at 70% and the final ABV will likely be somewhere around 75-77%. So your overall strength depends on where you stop the Spirit collection and let everything else go to the Tails jar.
OK this is getting too technical now, not to mention too long!!

Good luck
AM
