I've done this quite a few times. You will need to test as it varies for every brand/style/color in my experience. I usually end up somewhere around 90% base, 10% white, 1% activator. Sometimes no activator, sometimes more white, sometimes straight base, etc. It just depends.
Wash testing is crucial as well. Some 50/50s DC REALLY well and sometimes they suck hard and it isn't obvious out of the dryer which way they're going to go. I've gotten tonal looks on some blends with as much as 100% white and 6% activator that look like bright solid white before washing, but fade to basically nothing post wash.