I can highly recommend the book "The Russian Empire: a Multi-Ethnic History" by Andreas Kappeler.

He has uncluded many demographic tables including the 1897 census as well as earlier and later ones. In 1719, the book claims that 70.58% of the population was Russian (not including Ukrainians and Belorussians). Combined, they formed 85.86% of the population.

In 1989 Russians made up 50.80%, and Eastern Slavs combined made up 69.77%.

In the 1897 census this was 44.31% and 66.80% respectively.

I hope this is what you were looking for.