You don't want to use it on a painted surface, it will likely soak into it and cause to peel off later.
I used Red Devil lye crystals and never had an exact mix. Basically it's how strong you are willing to use it, what you want to use it on, and the risk of getting it on you. You need to use strongest for ground contact areas, such as soaking wood fence post bottoms on untreated wood.
I don't know if they still sell it, but there was a "no splash" or heavy clorox that already had lye (sodium hydroxide) added to it. It's probably strong enough by itself. I've taken the clorox and added lye till it wouldn't mix anymore, till all the crystals wouldn't dissolve, a fully "saturated" solution, but you have to be VERY CAREFUL WITH IT. That sort of solution is so strong it will instantly start to burn the skin off if you get it on you. That's the "environmentally friendly" mix I've poured into my septic drain pipe twice to clear blockages by built up muck over the past 20 years.
Normally for tree roots you'd put copper sulfate into the septic drain field line to kill the roots back a few feet, but that's restricted now and the cost went up 1000% following EPA restriction. The copper sulfate also didn't dissolve built up "muck".
I get lye in a 25 pound bag for that from the farmer's co-op and mix a saturated solution (about same as liquid plumber) in 5 gallon paint pail for that, then pour down a two inch pipe with a funnel I've inserted into the drainfield. I then pull the pipe back a foot at a time as I put the solution in to make sure the entire drainfield gets it. Works wonders to dissolve and clear the drainfield.
Probably a gallon of clorox with a half cup of crystal lye would be a decent mixture to kill mold, mildew and preserve at the same time. Consider that you can make soap from lye with a cup of fat, half cup of water and 3 teaspoons of lye, that's how strong it is.
If you wet the wood with the mixture and let it surface dry between each application it will get some down into the wood and help preserve it. It's not as good as pressure treated wood, but about the best you can do without using poisonous product which can remain for years. Lye and clorox eventually breakdown in the presence of acids, such as acid rain, or acid water from soil runoff, creating sodium salts. When the lye breaks down in the wood it's like driftwood soaked in the salty ocean and tossed on shore for long life quality in the outdoors, but not quite as good as actual driftwood which was thoroughly soaked clear through. Down South, especially along shoreline areas, people will use interesting pieces of driftwood in outdoor gardens. The driftwood has been in the salt water often for weeks or years and fully soaked with sea salts and minerals, bleached by the sun and then delivered on shore where it dries out and is bleached even more by the strong southern sun. Such true driftwood (not the imitation stuff so many sell) will last 10-20 years depending on sunlight and humidity conditions when left in a garden even without further treatments. Soaking wood with bleach and lye combination mimics that process.
A lot of what's sold today as "driftwood" really isn't. Much of it's not from wood which spent weeks, months or even years in salt water, then washed up on shore, but instead just weathered wood above ground contact, or imitation done by sandblasting to give the appearance of driftwood.
If you want to make your own little piece of driftwood find a small twisted piece of wood or interesting piece of twig you cut to fit, or even a cut out piece of a knothole which can go into a quart size Mason jar. Add bleach, full strength, add one or two teaspoons of lye, cap it, and swirl around once every day or two. In a month pour that off and rinse the wood under some clear water to clean solution off the surface and then let it dry. You will have your own little piece of preserved and bleached driftwood.