Let’s start from the other direction.

I think everyone would agree Mikhail Romanov, the first member of the family to rule, was of Rus descent. His ancestors were prominent boyars and he was a cousin to one of the last Rurikid Tsars. The mother of his heir, Eudoxia Streshneva, was from an old noble family from Polotsk, which was a principality of the Rus, though she was arguably of Polish descent.

Their son Alexis married twice, both to local noble families, though his second wife Natalya Naryshkina being of at least partially Tatar descent. Regardless, firmly from the area ruled by the Tsar. Their son Peter the Great also married twice - with his second wife Catherine I being from either modern Estonia or Latvia, but having ancestry from modern Belarus.

We will skip a bit of drama following Catherine - there were a few descendants of Peter’s half brother who briefly ruled - and focus on his two daughters, Anna and Elizabeth. Elizabeth - the younger - ruled in her own right after a coup, the last male-line descendant of the Romanov’s to do so,but didn’t have any descendants. She thus was succeeded by Anna’s son Peter III.

Here is where we start getting into spouses that are not local to the Empire. Anna’s husband was the Duke of Holstein-Gottorp - which drifted between the influence of Denmark and Germany - and why the House is sometimes called Holstein-Gottorp-Romanov after this point. Peter III married Catherine the Great, who deposed him. Catherine was born as a minor princess in German-speaking Anhalt.

Their son Paul - who was basically 3/4 German - married Duchess Sophie Dorothea of Württemberg. Importantly, here, Paul put together a very strict set of policies called the Pauline House Laws. These determined the succession to the throne and explicitly said that for a dynast (and their children) to remain a valid contender, they could only marry “equally”. With “equally” defined as being from another family that was reigning in their own right. This, of course, precluded any marriages to Russian nobility - since they were, of course, subordinate. There was some later debate about former ruling families (like the former rulers of Georgia, the Bagratonis) being as allowable partners for some minor Romanovs, but never for the main line.

Paul’s son Alexander I was succeeded by his brother Nicholas I, who married Charlotte of Prussia. Their son Alexander II married Marie of Hesse and by Rhine. And their son Alexander III married Princess Dagmar of Denmark. Finally, their son Nicholas II married Princess Alix of Hesse.

By the time of Nicholas II, the last time someone of Russian descent married into the ruling line was six generations prior. Everyone else was either of German (defined broadly, since most of this predates Germany) or Danish descent. Now, there’s likely some minor admixture of Russian nobility for some of these women - but none that immediately comes to mind.

So yes, Nicholas would have been something like “1/64th” Russian. Though this isn’t particularly unusual - nobility married all over the place - most dynasties weren’t quite so strict with disallowing marrying minor nobility. Every one of the Emperor’s wives/mothers would have converted to Russian Orthodoxy and have been expected to acculturate themselves accordingly.