[Steve Ruch]

What percentage of fermentables in a partial mash should be extract and what percentage should be base grain?

[denny]

What percentage of fermentables in a partial mash should be extract and what percentage should be base grain?

Whatever works.  I've never seen any standardization.

[Stevie]

The biggest reason to mini mash is to use grains that must be mashed to get their full goodness. The basic rule I‘ve seen is 1-2lb base for each 1lb of grains that must be mashed.

[Steve Ruch]

What percentage of fermentables in a partial mash should be extract and what percentage should be base grain?

Whatever works.  I've never seen any standardization.

I haven't either, which is why I was asking.
I'm working on a project that involves partial mash recipes, among others, and I've never done it.

[santoch]

I agree with Stevie, I've always used that ratio of 1-2 lb of base to specialty grains.  That is so that you have enough Diastatic Power in the mini mash to convert it all, erring on the higher side to be sure.

[dmtaylor]

Maybe 1.5 lb of base malt per 5 gallons seems about right, but it doesn't really matter.  Like the others said, there is no official standard mini-mash.  Could be a pound, could be 5 lb... it's all over the place.  I think when I was partial mash, I essentially started small then with successive batches worked myself up to all-grain over the course of a half dozen batches.  But most people these days probably jump from "extract plus a few grains" straight to all-grain BIAB or cooler mashing, especially BIAB because it's so friggin easy, a caveman could weave some netting to do it.

[a10t2]

I'd say there's no reason not to use as much as you can within equipment limitations. It'll be less expensive and arguably improve quality.

[Joe Sr.]

I'd say there's no reason not to use as much as you can within equipment limitations. It'll be less expensive and arguably improve quality.

This is your answer.  I would add that, when I have done it (and if I do it again) all of my specialty grains go into the mini-mash, so you need enough base grain to convert those.  The extract is there to get the gravity up to where I want it and I use the lightest possible dry extract (Breiss Pilsen, IME).