I appreciate you asked for a calculation but it would be more normal to use a correction table like the one published by Transport Canada.
You have asked for decision altitude, the current decision altitude would be a 260+1065=1325ft. The corrections are for decision height but they work the same way with altitudes. For 1500ft and -30C the correction is 280ft so, using the TLAR principle I would add on 250ft to get a DA of 1325+250= 1575ft.
Picky interpolation (which you would not do in real life) gives me a correction of 255.45ft.