This link is to an automatic painters stretcher calculator. All you need are the outside dimensions of the stretcher you want to build. Just enter the two dimensions and it will give you all the lengths. Remember that as with the other stretcher charts, when you assemble the inside frame this calculator assumes that the longer piece will be on the inside of the butt joint.
**The one exception is building a square stretcher where the calculator 'flips' one of the values. When you lay out the inner frame, check the measurements and make sure each side is 1 1/2" less than the final sides.
AUTOMATIC STRETCHER CALCULATOR