I would be more concerned about the ALT than the OAT wrt Start De-Rich setting.
Light it early rather than late as alluded to previous - as the RFM says. OAT versus Light Off N1 speed chart - 13% at those temps but never less than 12%.. You will want it to get hot otherwise it may stagnate.
Take some de-ice fluid as well. Was forced to leave a AW109E outside in -25C overnight once. Took me about 3 hours to get it going again.