wella highlift

Hi all I previously used Wella KP 12/81 and 0/81 to lighten my hair. During the numerous lockdowns I have reverted to Blondor and toning with CT as it’s what I had available but wanting togo back to special blondes but the 0/81 is no longer available. I am thinking of using 12/89 with 0/66...

Would I need to lower the developer down to 6% to cover the white ones? Usually I would use 12% at the back as she is Still very dark there naturally Thank you :)