In 1985, the Soviet Union had about 39,000 warheads. At most, perhaps 8000 of these could reach the US (this is a guesstimate based on the number of ICBMs, SLBMs and long-range bombers and an attempt to factor in MIRVed missiles; I'm too lazy to do the math). The rest were tactical warheads or on platforms which gave them the range to reach targets in Asia or Europe. So, maybe 20% of their stockpile would have been used on US targets.
The US was different; it had both a bomber, ICBM and SLBM capability which could reach the Soviet Union, but also a number of warheads based in Europe which could also do so. In 1985, there were 21,000 warheads. I won't hazard a guess on the percentage, but it would have been a lot higher than 20.