Converting part of a grain bill to extract (beginner edition)

[Hangdog]

Hello, all. I've moved from kits recently into partial mash BIAB technique I found on this very forum. I'm still limited by a 5 gallon mash and boil kettle, but most partial mash recipes I find require something bigger. I'm trying to make sure I correctly understand how to convert part of a grain bill into extract so that I can get the mash volume down a bit.

So, for example, I really want to make this Porter. Recipe calls for 3.5 lb (30.8%) CBW® Pilsen Light Powder (Dry Malt Extract), 4.5 lb 2-Row Brewers Malt, and around 3 lb of specialty grains. To reduce the mash volume to fit my kettle, can I just reduce the 2-Row to 1.5 lb and increase the DME by 1.8 lb (3 * .6 for the conversion), bringing it to 5.3 lb of DME total?

Would that get me about the same result? Or am I misunderstanding something about this conversion process?

Thanks for any help.

 

[BrewnWKopperKat]

In general, you will want to make sure that you have sufficient diastatic power in your mini-mash (this is a good place to start: http://beersmith.com/blog/2010/01/04/diastatic-power-and-mashing-your-beer/). And you can use that link to double-check my work!

Your adjustments look reasonable to me.

 

[chickypad]

If you want you can easily mash the whole 7.5 gals in a 5 gal pot. I brew 3 gal BIAB batches in that size pot and have gone up to 10 lbs of grain, I just have to do a quick dunk sparge to get the full volume. Mash with 3 gals, if you squeeze the bag we'll you'll lose about 0.5 gal to absorption, maybe a little more if you don't squeeze as hard. Dunk the bag in a bucket with 6 additional quarts, stir well then drain and add that sparged wort to the kettle. That should get you 3.75-4 gal boil volume.