Yes, it is frustrating at first because you only start knowing what's best with experience.
From what I've seen, you can leave the Ideal number of workers at 0 and that won't prevent the governor to try and hire workers when needed. I think it has mainly a niche use, such as if you've noticed recruitment is slow, you might want to plan it ahead to avoid a bottleneck. Look at your current amount of workers, look at the asset you intend to be building soonish and how much workers it needs, add up the numbers and set the slider accordingly. This way, when you eventually start construction, you're guaranteed to have the workers resource in stock.
As for the going rate, you can know it by looking on the bottom left info panel when you have the Zone tab opened and clicked on the Population number box. There are stats on the private sector economy, and among them the median wage.
I agree your proposal to set the policy as x% above the going rate would be much convenient, though also very much be fire and forget. I still lack experience to tell how quick the median salary can vary and thus how often you'd need to make sure to pay your workers better. But the rule of thumb is if they're not so happy and yet have food to eat, they probably could use a raise. If you can afford it.
Also there's at least one Stratagem to boost worker happiness (albeit at the expense of the general population one, but if your goal is to attract workers to the public sector, it's probably not that much of a drawback).
To try and sum it up there seem to be several ways to manage this problem.