Try to determine the values X, R and Angle of both sensors as good as possible and use it as an input for the press brake controller. Then make a test bending to 90,0 degrees.
Measure the result angle with a separate tool at the same position as the laser measure the plate. If the result is for example 91,0 degrees you have to reduce your mounting angle settings of 1 degree (front or back sensor), for example 39,0 degrees instead of 40,0 degrees. Then test again. The result angle should be now 90,0 degrees.
The parameters X- and R-position are not that critical. An accuracy of 0,5mm is sufficient. Both parameters refers to the centre of the laser beam lens. Please have a look at the technical drawings.