4.5 - 5 hrs for 20lbs ??? That's all ......?? Temp 175 in dark correct ?
Epicurious has a great chart for turkey doneness, taking into account turkey size, oven temp & time: http://bit.ly/dMQo7M But it's best to take the internal temperature too -- for a juicy bird, you'll want to hit at least 150 in the fleshy part of the thigh (the temp will continue to climb as it rests).
Hi almost all foodpickle members are in the US so we have US measures. The advice is from Martha Stewart website. If you are in the UK use this guide:
150F would be the right temperature tested in the thigh. However also be sure to tent it covered in aluminum foil for about 20 minutes once it comes out of the oven because it will get a heat boost after it comes out of the oven and is resting.