Hello, I need instructions on making the gravy for chow mein. My last attempt was a three gallon disaster. It tasted really good, but I couldn't get the consistency right. The corn starch wouldn't thicken and it kept clumping into gooey balls. Was the corn starch too old? Did I not cook it long enough (about 20 minute boil)? How much corn starch should I have used for three gallons of gravy? Thank you!