All well and good but different shaped joints need different cooking times - long and thin cooks more quickly than short and fat even if the weight is the same; if you are cooking meat on the bone it cooks differently to meat off the bone, and the size and shape of any bones will also affect cooking times...and are you sure your oven temperature is correct?
Bollocks. If that were all true, the flapping of a hummingbird wing in Venezuela would determine whether my tea in the morning is piping hot or all a bit Parish.
My oven is Neff. It is infallible.
That said I do find that looking at the food to see if it looks done helps an awful lot. Especially that time we had a power cut.