A relationship shouldn't be a matter of similarity. It should be a matter of mutual will instead. Tastes and hobbies, even some cultural beliefs are subject to change in time, a relationship based on those kind of similarities is not destinated to last long.
A more relevant topic would be character similarities, but even I this case I'd say it's not really important to find a similar partner, it's more important to find someone with a character you like, even if it's very different from yours.

if i had to random guess it....

maybe 70% overlapping interests with intensity levels different.

If you are too much alike you end up doing EVERYTHING together... or feel bad for excluding something from something they love too.

People need their alone time, or even there... just not near someone specific time.

I have no significant other, but I'll still answer.

A couple should have similar entertainment activities, so they would be able to do things together. I couldn't see a real relationship working if a guy hates movies, and his girlfriend or wife watches movies all the time. Religion is also important. A legit Christian guy is never going to date an atheist and vice versa.

As for personality, I think they should be different. You can't really bond with anyone and except them for who they are if they are almost a girl/boy version of you. This ties into the entertainment activities thing. I don't think it would work out if you do absolutely everything together.

80% similar.

I'm not very similar to my GF (religion, political views, intelligence, sophistication, interests, stance on family matters, sexual preference) and honestly that's a big problem.

It's much much better if you're similar.

You should have some similarities but also some differences. It's good to have some common interests because that's how a lot of people meet.
With religion, I think for some people it's just a personal preference. Personally I would want my girlfriend to have the same faith as me. Other people aren't quite so picky. I think is best to have the same religion, so you don't have that element that could possibly create conflict.
As for personality, opposites attract. Introverts go well with extroverts, although it doesn't need to be that way, and people of any personality type can probably do well together if they are both willing enough.
Some common interests are good, but some different interests are also good.

Mostly

Women should be a bit on the feminine mental side.I don't think the dominant female relationship works.Also Men like to have Women of more care and pure mind.

Men can be a bit naughtier and should be a little more dominant.

Religion and ethnicity and Race has to be same.I don't buy the multiculturalist shit

Entertainment also similar but can have a little bit of difference due to gender

Intelligence has to be close as it is required for mental connection