Berserker n' Assasssin Mix?

You always hear people talk about mixing the two, but I can't seem to find any information as to what is the ideal ratio of each.

Not sure, but I'd assume Assassin's until your crit chance w traits, fury, food/utilities etc is 100%, the rest Berserker.

It varies based on your trait and what kind of consumables and upgrades you want to use.

Well, Berserker's and Assassin's attribute combination is Power, Precision and Ferocity. The only difference is that Berserker's major attribute is Power and Assassin's is Precision. Ferocity (damage when hitting critically) will be the same, no matter which armor you pick. So what you balance by mixing gear is Power (flat damage increase) and Precision (chance to hit critically). In the best scenario, you go full power and get more Precision through other means. There are several traits and skills that give Precision, either by yourself or by other classes with boons. The meta builds don't use Assassin's because they assume that enough crit chance comes from the Druid (with Spotter trait) and Warrior (Banner of Discipline) and your own skills/traits. So if you have a Druid and a Warrior in your team, you can use full Berserker's and still have 100% crit chance in most scenarios.

If you play solo, balancing crit chance and power is more of a topic. If you exchange too many gear pieces for Assassin's you will hit critically more often, but with lower base damage, so the total result will be worse than if you had kept one more piece as Berserker's. There's also a Sigil of Accuracy that increases crit chance by 7%. Sigil of Air is about 4% of my DDs damage, that's not really much. I might get more damage in solo play (no boons from others) by replacing it with a Sigil of Accuracy. In fractals, if you use fractal potions, you'll have more base crit chance (depending on your AR), so Assassin's gear would be useless in many cases (because more than 100% crit chance is not possible). In that case, if you have a Druid and Warrior in your team, plus your own sources for Precision (that can also be food or utility item) you are better off not having Assassin's at all and even replace one or two pieces of your Berserker's gear for Valkyrie. You would get more health that way, and more health means better uptime for the last Rune of the Scholar benefit, which adds to your damage (in addition to higher survivability).

There are a lot of different factors that come into play and you would need to weigh them according to the situation. In perfect play, you would change gear, skills, traits, food and maybe even Sigils depending on your group's composition. You would try to get 100% crit chance but not at a cost of too much Power. In reality, you just gear for group content because even if that's not max dps for solo open world play, who cares? Solo open world play is so easy, you don't need to max your dps to have a good time. Most people don't even use food or nourishments for open world play because you faceroll everything without it. And with a group, you are totally fine if you can create 70% to 80% crit chance on your own. My DD gets Fury basically permament, so that's +20% crit chance, Sigil of Agility gives another 8.5%. If I group with a Druid (+7%) and a Warrior (+8%), I only need to gear for 56.5% crit chance to have 100%. Now I have several choices, I could swap some Berserker's Gear to Valkyrie, I could replace my Sigil of Agility skill with something else etc. It's really up to you how far you want to go with perfection, but if you play Thief, you can really go without Assassin's gear.

As other people said I think it depends on what percentage of critical hits you want. I have a similar-ish build on my ranger where I mixed 2 different sets (Rabid and Sinister if I remember correctly) and on that I decided I wanted 50% critical hits. But that's just something I put together for my own enjoyment when I'm playing solo, so there's no need for it to actually achieve anything beyond being fun for me to play.

As a rough guide I'd say think about how important crits are to your build. If you're using a lot of traits/skills/sigils etc. that give additional effects on critical hits then think about how often you want those to trigger (approximately, it's random so with 50% you could get 3 crits in a row then nothing for the next 7 attacks, then another 2 etc. but it will even out to 50/100 overall).

If it's not important how often you get critical hits then it might actually be better to just go for Beserker's.