Aargh turkey calculator experts please

[backonthemerrygoround]

Okay, so I have a 5.9kg turkey. Say 6 kg for ease of calculation.

BBC goodfood etc say 20mins per kg + 90 mins.

I work this out as 3.5 hours? ( (20 x 6 = 120) + 90) = 210 = 3hrs 30 mins???)

Turkey wrapper says 4.5 hours?? This will throw my whole lunch timings off!

HELP. And calming thoughts please.

According to Delia, 30 mins at 220c, then down to 170c for3 hours. Then final 30 mins, uncovered at 200c. That's for bird up to 4.5 kg.
7-9kg bird 45 mins high, then 4 hours low, and up for the final 30.
Hope it works out, happy Xmas