You can use a protractor or some sort of angle finder, like the one pictured below, to help dial in the angle you are looking for. If you don't want to purchase anything, trial and error may work best to find the angle you seek.
You will find that you will have to "over bend" the axle to achieve the angle, like most materials, they want to retain their original shape. The axle will spring back slightly, i.e. if you bend to 10° and release you will find the axle to only have an 8° angle when finished. This is only an example. Work up to the angle you want. Straightening the axle after bending usually does not yield good results.