The “rule of thumb” is if you reach a level and then take no more Vitamin D or get any sunshine, your levels will decrease by half every two weeks.
So when you took an average of 6600 iu, your level was 108ng/ml. So I would guess and say if you took half that amount or rounded to 4000 iu a day, you should end up at around 50ng/ml. We usually recommend 1000iu for every 25lbs of body weight.
If at any point in the future you got sick or extremely stressed or something taxing on your body, you can take extra Vitamin D to compensate.