Use the 27 J at -20ºc, being metric the conversion is already done for you.  In testing, you should have no problems what so ever passing impacts at this temperature provided you use sound welding parameters.  Remember, if impacts are required by constructing code or specification, you have to actual DO the mechanical tests, you can't just take credit for what the filler metal specification provides you.  This is a very good and very forgiving filler metal just use good practices when welding test coupons.

Not really sure what your asking but if you're trying to certify weld material the requirement is to run 5 Charpies toss the low and high and then average the remaining 3.
Your results can be stated in either one, though you have to think ahead since they are not exact. But then neither's the temp control when testing these things.
So if you use C you use J. If you use F you use ft/lbs.

I'd like to thank all of you who gave me some hints, but I'm scared about the item 1.3 of SFA 5.18 which reports : each system must be used independently of the other without combining in any way when  reffering to welding metal properties. In my opinio, I could only use 20°C if the rod was ER48S3 as it is  ER70S-3, I think - I'm not sure- I should us 27J at 17,7 °C. Do you agree with me? The test should  be approved. Take a look at the item 1.3 of SFA 5.18, unless  O°F is  20°C regarding conversion from US Customary Units  to US Units in this case you're right Jon 20013.

From now on we'll be good friends, for sure. That's the friends apart.
best   wishes