Beef cooking times

[rapscallion]

I have a 3kg piece of topside. I want to cook half well done (for the heathens) and half rare (for me!), so will split the joint in half - 2 x 1.5kg pieces.

I am slightly confused on the cooking times. What weight do I use in the roasting calculators? 1.5kg or 3kg?

This calculator gives 1 hr 6 minutes for rare for 1.5kg, and 1 hr 40 minutes for well done.

But the times for 3kg is 2 hrs 13 mins for rare and 3hrs 20 mins for well done.