Can’t remember been 2 years when I was working with a small kettle. I think I aimed for 1.050 in carboy 1 and added water to hit about 1.035 in the other or something like that. A 5% and 3.5% if I remember correctly
To follow up I even did it as a parti-gyle cheat. I split into 2 small car boys and added water to one to get an ordinary bitter in one and stronger bitter in the other.