Divide this temperature by 14*--the amount of temperature drop that 1 CLO gets you--to get the CLO needed to reach that temperature. (For 15*, that would be 69 / 14 = ~4.93.)
Divide this CLO number by the CLO/oz/sq yd rating of your insulation (for Climashield APEX, the only one I remember offhand, that's 0.82/oz/sq yd). (For 15*, with APEX, that would be 4.93/0.82 = ~6.01 oz/sq yd. So, 6 oz/sq yd APEX [or two 3 oz/sq yd layers sewn together] should get you to 15*.)
Note that the above doesn't take humidity effects, wind, hot/cold sleepers, or drafts into account. It assumes a "perfect" system. But, it's a good ballpark.
Hope it helps!