I've never heard of boiling the Sta-Flo, but decided years ago that I would rather pay for the prepared Sta-Flo than to boil my own starch, and also don't have to keep Sta-Flo in the fridge!

I use the Sta-Flo liquid starch too (usually 50:50 mix for piecing), but I've never boiled it...just used tap water. Why do you boil it?

Homemade Spray Starch: Boil 1 quart of water. Dissolve 4 tablespoons of liquid starch into 2 cups of cold water; add to boiling water and return to a boil, then remove from heat immediately; add 2 more cups of cold water -- let it cool. Starch should be clear and not milky. Place it in a mist spray bottle and store extra starch in the refrigerator. Clean the iron with a wet terry towel or dryer sheets if starch starts to build up on your iron. This is extremely cost-effective--only 2 cents a quart no more clogging or inconsistent spraying!
When my first attempt at the star was stretching, Margo suggested to starch my fabric more. I made a heavier batch of starch, mixing half Sta-Flo and half water, following the same process as above, this batch was somewhat milky looking.

i.e. this is not my recipe but I have been making it so long that I do not remember who to give credit to, sorry.