How do I adjust cooking time when temperature may be off by 5-10C? Over the holidays, my mom tasked me to prepare a Crown Roast of Pork for our Media Noche (basically it's the meal you have as you approach the New Year with fireworks)  meal. All of the recipes I found on youtube used the Fahrenheit. The procedure would go:


Preheat Oven to 450 F
Roast Crown Roast for 20 minutes
Reduce heat to 350 F and wait for another 130 minutes 


Our Convection Type Oven has a Celsius Degree Scale. Converting those temperatures to Celsius, I got:

450 F = 232.2222 C ~ 240 C
350 F = 176.6666 C ~ 180 C


Now, the dials (yes, our oven has an old school rotary dial for time, temp, and function) are only divided into 10s, from 50 C to 250 C, so I didn't have a clear 232 and 176. What I did was I rounded up to the nearest tens instead and then used the same cooking time, thinking that the Fahrenheit equivalent of the rounded up Celsius values wouldn't be too far off. 

When I took the Crown Roast out, the outside was charred a bit but the inside was cooked well, although a bit dry. My siblings described the char to still be edible and didn't have that bad and bitter burnt taste.

So, after almost botching our dinner, I rescaled my values back: 

230 C = 464 F (originally 450 F), cooked for 20 minutes
180 C = 356 F (originally 350 F), cooked for 130 minutes


Now, we see that there is quite a disparity between the new Fahrenheit values, and being cooked at a higher temperature dried up my pork and burnt the outside. 

My question is, is there a "time-scaling" rule to follow when converting Fahrenheit to Celsius? What I mean by this is that for example, in my case, it might be:

450 F for 20 minutes or 230 C for 25 minutes or 220 C for 18 minutes


Wherein after converting the temperature from Fahrenheit to Celsius, the time you cook it in that temperature would also change since the temperatures or not the same. The temperature may have been decreased or increased in order to accommodate the limitations of the cookware. 

Sorry if this winds up being an opinion based question or something that can only be discussed but not answered. As much as possible, I'd like to see if anyone tried something with regards to this, may it be from experience or from a book or a class or a show and if others have used the same method.
Rounding to the nearest 10C is more accurate than your thermostat probably is anyway (don't round up, round to the nearest). 

Conversion isn't your issue, your thermostat is much more likely your culprit.

Use an oven thermometer, not your dial.

And keep in mind that ovens hover above and below their set temperature by switching on and off.

Use Google or whatever to do your conversions, and then your oven thermometer to keep your thermostat honest. The natural variances you'll get from rounding are fine.

You should also be using a meat thermometer; pull a roast to rest when it is 5F below your target temperature.