How do I adjust cooking time when temperature may be off by 5-10C?
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.