I have been having trouble hitting my target gravities in my partial mashes so have decided to use some simple math to help me out (my teachers were right, math is useful).

Does the following seem right? Is there a simpler formula?

My partial mash yields ~3 gallons of wort. So If I were to take a gravity reading and divide it by two, I would have an accurate measurement of the partial mashes contribution to the total gravity, or is it not so simple?

To then calculate how much extract I need to add, I would subtract that number from my target gravity to determine how many gravity points are missing (the points that the extract would need to contribute)

As Liquid Malt Extract will make a wort of ~1.034 specific gravity per pound of extract per gallon of water, I would then divide the missing gravity points by 34, and multiply by 6 (the batch size) to find out how many pounds of extract to use.