Research.

[stormbringer3]

What's a recommended percent to use?
Thanks for any opinions.

[EwaldvonKleist]

I always use 12%. I haven't done any experiments on the correlation between research investment and research progress, but imho the advantage from inventions is big enough that it pays off to be ahead. Especially with key techs like turbines, BB turret layouts and DD and torpedo techs.

[NimrodSam]

I use 12% except when up to date on everything. Then I set everything to low except guns and reduce to 9%-10% until January. Doesn't seem to do any harm; I often get more techs anyway. Harder to tell whether it has gained anything on guns.

[varsovie]

Since research takes time to get and are important, I try to maximise the % devoted to it.

But...

Since they're not guaranteed in a definite timeframe and most aren't useful rigth away (eg you still need to (re)build a ship tu use better engine), I tend to lower it at the eve of war and use the extra income to build DDs or get a couple ship out of mothball.

[stormbringer3]

What priority settings do people like? I usually use all set on low with about 4 set on high. I'm open to better ideas.
Thanks for any opinions.

[EwaldvonKleist]

I don't know about the detailed research game mechanics. My fear is that, if you have researched the highest tech level possible for a certain year, the game continues to invest research in this category and thus wastes research effort before the next technology is unlocked.
Therefore, I keep everything at medium except for a few high priority techs so I don't run into situations where I have maxed many categories and waste effort.
Since I prefer big gun fights and the time 1900-1940, my priorities are machinery, fire control, turrets, ship design and guns. But this depends on the nation, if you play Japan you may wish to focus on light forces or aircraft, and as weak nations you may focus on light forces for sea guerrilla warfare.

Everything else stays on medium, except for fleet tactics since some of them tend to confuse the AI and make fleet handling harder, and lighter than air, since all I want from airships is recon and they can do this satisfactorily really early.

This being said, you can probably do a lot of minmaxing if you fully know the mechanics, which I don't yet.