A ballpark estimate for the number of Russian immigrants to the U.S. from 1975-2010 can start from the roughly 350,000 Russian born people in the United States in 2010 (interpolating census figures), which needs to be increased by the number of people who immigrated in that time period but subsequently died or moved to another country in that 25 year period.
From 1975 to 1990, almost all of the Russian immigrants to the U.S. (except a handful of defectors and refugees) were Jews under a policy that allowed about 250,000 Jews to leave the Soviet Union, most of whom went to Israel, but some of whom went to the U.S.
The remainder, in the time period from 1990 to 2010 were not exclusively Jews, but family ties made Russian Jews more inclined to migrate to the U.S. than Russians who were not Jews (important exceptions would be "mail order" Russian brides and orphans who were adopted by Americans).
Realistically, given the OPs estimate of the number of Jews who migrated to the U.S., the percentage could be close to 100%, but probably was not that high.
Russian adoptions started around 1990, peaked in 2004 at 6,000 per year, and ceased around 2014. In all, about 44,000 Russians, overwhelmingly non-Jewish, were adopted by U.S. citizens from 1990 to 2010.
There are about 10,000 foreign brides who come to the U.S. to marry Americans each year, but almost all of those from Russia would be in 1991 or afterwards, and the percentage of the total that is Russian isn't the easiest thing to discern. Some Russian brides would be Jewish, but the lion's share would not be. A high end estimate would be about 2,500 Russian brides a year for 20 years for a total of 50,000, more than 90% of whom are not probably Jewish.
Assuming that 90,000-94,000 foreign born Russians were not Jews and that perhaps 500,000 people had immigrated to the U.S., a third of whom had died or remigrated since 1975, the Jewish percentage would be roughly 80% with a heavy bias towards less recent Russian migrants. This is a high estimate and the actual number could be lower, as it assumes that almost all migrants from 1990-2010 who we don't know weren't Jewish were Jewish. But, it would be hard to argue for a higher percentage.
This estimate is really only a back of napkin figure, however, and is subject to considerable uncertainties, not the least of which is that the U.S. does not keep track of the religions of people who immigrate to the U.S.
This estimate is largely confirmed, however, by analysis of the question by several different scholars using different methods and estimates were in the vicinity of 70% to 85%, depending, in part, on whether people in households which have some members who identify as Russian Jews, who do not themselves self-identify as Jewish (e.g. an Orthodox Christian spouse or uncle), count as Russian Jews, with a more expansive definition associated with the higher percentage and the lower number associated with a narrower definition.
This source notes that there is considerable regional variation in the percentage of Russian-Americans who are Jews. Russian Jews tend to be concentrated in the Northeastern United States. Other Russian immigrants are more likely to live in major cities in the Midwest. And, a significant Russian Christian minority sect migrated almost entirely to California. These kinds of immigration patterns driven in part by historical accident aren't unusual (e.g. in a similar vein most Basque Americans have ties to Idaho and Denver received lots of Jewish migration in the 19th century because National Jewish Hospital was founded there to treat lung diseases like TB which had been a problem in the Northeastern Jewish community).
