I don't think there is any one "perfect" lye solution concentration for all recipes all of the time. You have to know your recipe and choose a reasonable lye concentration. Some lye solution concentrations will work better for some recipes than others. I'm finding differences in how my soap batter behaves with just a 1% or 2% change in lye solution concentration, so you don't need to make a big change in the concentration to get distinct differences in behavior from a given recipe.
If we're talking about a 100% olive oil recipe that will saponify slowly, I would use anywhere from 33% to 40% lye solution concentration (this is NOT water as % of oils). I would use whatever lye solution concentration I felt most comfortable using within that range -- some people are uncomfortable with 40%, but okay with 33%. Less water = faster trace time. Experienced soapers even use up to 50% lye concentration, but I don't recommend that for a beginner.
For a recipe with a balanced blend of fats, I might use anywhere from 30% to 33% lye concentration. Higher concentration means less time to trace, all other things being equal. That's fine if I'm making an uncolored or solid color bar and want to "get 'er done". Lower concentration means more time to trace, and that works better if I want more time to do whatever fancy decorative work I am able to do.
Lower concentration also means a greater tendency for the soap to go into gel (and possibly overheat) on its own without additional heat. There is also more tendency for the soap to show streaking or mottling. If any given recipe traces too fast, I might try reducing the lye concentration by 1% or 2% and see if I get more time before the soap comes to trace. If the soap shows streaking and mottling and I don't like that or if it tends to gel and overheat easily, I might increase the concentration by 1% or 2% and see how that works.
For a 100% coconut oil soap that saponifies quickly, a 28% to 30% lye concentration would be fine.
That said, I seldom use "full water" (about 28% lye concentration) because the soap batter may not stay emulsified with that much water, especially for recipes with liquid oils (vs. lard, tallow, palm, PKO, and/or coconut). If the emulsion "breaks", the batter will separate in the mold into fat and lye layers. That is not a good thing. Also a high-water soap can tend to be too soft when I want to unmold at 12-24 hours, again this is more likely if the recipe contains liquid oils.
Related threads:
What is wrong with my soap? and
lye solution/concentration vs. water as % of oils
