Easiest way I've found is to find a level hard surface with a wall. Place your front tire against the wall and measure to the tip of saddle, center of the bars, and center of BB. Next take measurements from the floor to the center of the bars, top of saddle and BB. With these measurements you can calculate the stack and reach of the center of your bars(X/Y measurements)from the BB, and the height and set back of your saddle from the BB.
When you get your new frame measure both ways to the BB and adjust the others to make up for any differences between BB's between the new and old frames.
The important thing with using this method is that you use the same floor and wall to measure along with the same wheels and tires.
Not necessary to have same tyres & wheels. All that matters is that the wall & floor are vertical & horizontal and the relative positions will be the same.