What I find is that for an approach to a rig commencing from 500' AMSL, start slowing down for a straight in approach at 40 seconds to run off the GPS, fairly positively, usually the power is down around 82-84% N1 (in the C model). Hold 500' until the profile fits, lining up the end of the pitot tube with the helideck and then start your descent.
One interesting thing I find quite consistently is that (roughly) your Radalt height equals your groundspeed X 10. Ie at 400 AGL/GS 40 Kts, 300 AGL/GS 30 Kts etc. ROD is bang on 500FPM. I use the far side of the helideck as my aiming point for the approach angle, this seems to bring me in over the pad not too low and not too high. Torques are usually well below 50% until coming over the deck so I think no probs landing on should an engine fail after the LDP.
Try it and let me know what you think.