Nailing down an Oktoberfest

Northern Brewer General
1

Heres what I have so far for a 5 gal batch

7lb German Vienna
5lb German Munich

Tett hops at 60 to = 23IBU

Bavarian lager 2206

I want to start at 145F and rest for 30min
Then take a thick decoction and boil for 20 min and add back to get 160F
Then take a thin decoction and bring to boil to get up to 170F

Does anyone have a reliable way to figure out the volumes of the decoctions so I can at least get close to the numbers I’m shooting for?
I normally do 1.25qt/Lb water/grain ratio but with the decoction thrown in there should I modify that?
Should I still be doing a (batch) sparge like normal after all the decoction stuff?

Thanks for any help

2

For my only decoction mash I just used Beersmith’s steps and I hit my two decoction temps but missed the mashout temp by about five degrees somehow. Oh well, overall it went pretty smoothly and I got a terrific efficiency for a big beer. It’s cheating a bit, but that’s what technology is for. You could download the free trial if you don’t have it.