Thanks for all the help
I wrote three little programs for my TI85 calculator
the first program
you input, preboil gravity, preboil volume, and final volume
and gives the OG
the second program
you input, target gravity, target volume, preboil volume, and preboil gravity
and gives me lbs of DME to add, or the final volume needed to hit the OG
the third program
you input, final volume, target gravity, preboil volume
and gives the preboil gravity need to hit the OG