Randy:

I don't find it now, but my preference is 60%, in other words a 0.60 single would have 0.36 (or a little larger) engines on it.

Others double the original. A single 0.60 would be replaced with two 0.60 engines.

The 60% rule generally gives about the same performance as the original plane with its single engine.

Anywhere between works.

Bill.