Does the Uchiha:Senju DNA ratio matter in awakening the Rinnegan?
Answer given by @Ero Sɘnnin in the comments.

Not every Uchiha is able to awaken the Rinnegan. Nor can any common Senju.

It is not the Uchiha:Senju DNA ratio that governs the Rinnegan's existence. It is the Asura and Indra chakra mixing. 

Also, I think the chakra ratio doesn't matter as we see in Madara's case. Although more chakra can maybe speedup the process of Rinnegan formation.

Related: What if Hashirama had implanted Madara&#39;s cells into his body?