I use a brewing slide rule to calculate partial mashes. You can go to brewer's friend, or another online calculator. Plug in the recipe, then make the changes you want, keeping the OG and IBU the same. Strike water temp is about 10-12 degrees above your mash temp. Use 1.5 quarts per pound of grain. Base malt is 30 points per pound per gallon, DME is 42. So a four pound mash reduces the DME three pounds.