Freshen-up Shampoo for Dyed Hair?

I dye my hair about every 6 weeks or so. I use Clairol 35G and 30 Developer - two ounces of each. I also add 20 drops of Gray Magic. My questions is a mid-dye freshen up with shampoo. I am thinking I could mix 1/4 ounce of the dye, and 1/4 ounce of the developer and shampoo it in. Does anyone have any thoughts or advice on this subject?