I mix to about 20% but I expect the developer is higher than that as I use it fairly warm most of the time. I figure that I replenish at 20% and the older developer is maybe getting up to 30% at most, and when mixed, it probably produces about a 25% mix or thereabout.
I haven't seen any speed loss using that approach, and I don't have problems with crystals, so I figure things are working fine. I used to use a more saturated developer, and I felt that having to deal with saturated solution was a hassle, so I backed off the mix a little. Works well for me, anyway.