Does your Jepp chart change the Grid MORA change when it's cold? No? Then you should 'amend' it.
Any cold error correction is going to be a rough guess anyway as you don't know (normally) the ground elevation which can be around 10k over the Greenland Plateau or sea level with 10k peaks along the coast and the exact correction is dependant on the column of air height, and the temp at the ground station (which you don't have).
As a quick and dirty rule of thumb when heading down, i'd look at my ISA D, times it by the published MORA (in thousands) then times by 4 and add that to the published MORA.
ie. ISA-20, published MORA 12k. 20*4 = 80. 12*8=96. Add the zero=960. I'd level out at 13k and once the ECAMs sorted and follow up plan of attack, then refine further by taking the temp lapse rate for Greenland, and multiplying it by the air pressure of the aircraft. Then add 1 to this result and divide by the sum of the lapse rate times the pressure at ground level times 15. Take the log of the result, Multiply by ISAD and then divide by the lapse rate. Add this to the MORA.
PS. I've assumed Jepp's Grid Mora, giving 2000' clearance in Greenland. And if you want references, check the bit in the Jepps where is says to correct altitudes in cold weather, referencing the (obsolete?) ICAO 8186 in all circumstances except radar control... Airbus have similar opinions, giving low altitude calculations and a high(er) altitude temp correction graph, more fine tuned but something you can't commit to memory.