The Cottage Smallholder

I am a total novice at roasting beef – until yesterday I think I'd done it once. So we decided that for a treat, since we didn't have anyone else to please, we'd have roast beef for Christmas lunch. S bought a 1 kg fillet roast from the butcher (planning on leftovers!). We also have a digital thermo-sensor, so I did a bit of googling and the consensus seemed to be that if you want your meat rare, the internal temperature should be 49-52C, and that the temperature might rise by another 2-3 degrees while it was resting. Estimates for the amount of time roasting would take ranged from 40 mins to an hour. Whereas the butcher himself said 20 minutes in a hot oven.

Took the meat out of the fridge well in advance, preheated oven to 220C, stuck the sensor in the meat. At this point it read 15C. I set the timer for 20 minutes – at which point the sensor was at 27. So we gave it another 10 minutes, at which point it had reached 38C. We checked the meat, and all the evidence of our eyes and a skewer said that it was cooked. So we set it aside to rest while we did the Yorkshire puddings and finished the veg. After 15 minutes, the internal temperature had risen 14 degrees to 52C!

So we were glad we took it out when we did, because it was absolutely perfect, the best rare roast beef I've ever had. But it makes me wonder how you are supposed to judge it, if the temperature can increase so much after you've taken it out of the oven. If I'd trusted the temperature reading and my googling, instead of the evidence of my eyes, it would have been horribly overdone. I've googled some more and not really found any answers.

So any roasting experts here? How do you determine how long to cook it so that it's just right? Should I have cooked it at a lower temperature?