As sawbones said, 70% N1 is unusual. As long as the wiper bolt isn't picking up vast amount (70% is not required for light icing) then 55% is enough.
Only went to 70% once in 5 years of operating the -200 around europe.
BTW - If you are high on the descent and you need icing on, go as fast as you can (turbulence permitting). The TAT rise may be enough for you to reduce N1 to 40%...