The proper way to wire a fuel pump such as the Bosch 044 is to take a direct feed from the battery into a correctly rated relay and cable then use the existing feed from the old fuel pump wiring to act as the trigger for the relay. I wouldnt use any existing pickup points anywhere.
This way provided it is correctly earthed it should always see the voltage that the battery does - i.e 13.4v+ when running. It is vitally important that it gets the best voltage supply you can give it otherwise when you are driving the car under heavy load/high boost the last thing you want is for it to start to fail in its operation which could potentially mean you fueling could lean off.
Voltage fluctations @ idle wont make any difference at all to the actual idle of the engine...unless you're fluctuating between say 5v and 13v. Bear in mind the pressure at the rail is regulated by the fuel regulator - if your car is having issues maintaining a base 3/4BAR fuel pressure at idle then its going to struggle like hell on boost/higher up the rev range. You'd probably find that even at 8-10v the pump will be able to maintain 3/4BAR fuel pressure @ idle.
Hope this helps
Yep really helpful thanks - I'll get on it tomorrow and do it that way makes perfect sense now I have read your wisdom
Great stuff thanks for your help guys!