I don't think Mary Ellen's Best Press is starch. (Starch is normally made of corn flour, but potato flour and others can be used as starch too.)
Edit: With spray starch, you need to wait for it to dry (or at least soak into the fabric so it's not just sitting on top of the fabric) before ironing so you don't scorch the starch. With Mary Ellen's Best Press, I don't think you need to do that.
To starch yardage, I mix up a solution of Sta-Flo liquid laundry starch and water, "paint" it on with a large wall-painting brush until fabric is saturated, toss in dryer, then iron with steam. It's a pretty fast method, plus my finger doesn't get sore from spraying.