i want to feed my rats mostly grain mix(like the hard mix from pet stores but better!) , and the store bought stuff is no good ,

what is a good homemade grain mix i can make for my rats that be best for them or will meet most of there nutritional needs?

You can make subees grain mix and supplement with a high quality lab block or a dog food and veggies.
http://www.ratsrule.com/diet.html

mine mix thing is:
oat,corn,sunflower seed,hamster pellets

Our rats get Kashi, which is an unsweetened, puffed cereal made from several grains. It's pretty good & I snack on it too. We add other whole grains to this, usually millet, buckwheat & whatever else is available from the natural food store. This makes up about 80% of their dry food, the rest is a high quality vegetarian dog kibble. They also get fresh veggies & fruits, whole grain bread or toast, vegetarian dog biscuits and usually a bit of whatever we've eating unless it's unhealthy for them. We use sunflower seeds as occasional treats but they only get human-quality seeds (raw & unsalted). Everything is organic when possible.

Our rats have been healthier since we've stopped buying commercial rat food. It's been more than a year since we've seen a myco flare up & all 3 of our current rats are over 2 years of age (1 is nearly 3).