I've always replenished Xtol, and before that ID-11 (D76), ID-68 (Microphen) etc. I make up 5 litres and use 2.5 litres as the working solution and the rest to replenish it.

The advantages are far better grain, sharpness & tonality once the working solution has ripened, in fact the results are closer to using Xtol diluted 1+2.

Ian