How long do I cook a 0.810kg beef joint for?

[Nbg]

I'd like it done medium.

If it's on the bone you need 12 minutes per 500g + 12 minutes extra at the end of cooking.

If it has no bone, you need 15 minutes per 500g + 15 minutes extra at the end of cooking.

If it's stuffed you need 20 minutes per 500g + 10 minutes extra at the end of cooking.

It's not on the bone so it will need half an hour? Is that right?

More than that because it's more than 500g - I'd say 25 minutes + 15 minutes at the end, so 40 minutes.