My biggest beer went from 1.142 to 1.024, so about 17% IIRC. I used WLP037 (Yorkshire Square), and stepped it up twice. Beer 1 was a 1.040 brown ale. I used about a third of the slurry from that to brew a 1.060 Old ale, then I pitched my 1.142 barleywine on the whole yeast cake. I oxygenated with O2, then hit it with a second dose of O2 about 18 hours later. I did an open ferment in a bucket, but that was primarily because of the yeast strain I was using.
One thing I would strongly recommend is to produce a wort that is highly fermentable. Mash long and low, and consider using simple sugars in the wort.
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