easy...mark the centerline by placing the same diameter pipe next to your old 45 deg bend pipe and scratch it whith a sharp edge flat bar of steel or alu. it only scratch on the highest spot on pipe so the centerline is now visible. now take an adjustable angel swing and set it on the same angle as the bend pipe by placing is aganst the outside of the pipe. Once the angle is correct you can place is on the scratch line on the top of the pipe. And voilà now you can see the intersectionpoint of the centerlines. now you can mark distance of for example 100mm out of the intersection and repeat the same at the other bend. now you can measure the distance between the two marks and add the 2 distances of 100mm. For bending the new pipe, mark a certain distance and check distance again after complete the bending. Now you now what is the extra length you have by bending. Now extract that extralength from the distance you need for the next bendmark. I hope ist clear my English is not that perfect. greetings from Belgium.

Have any formula please give