Founder salary at pre-seed stage? (raised 500k)

General rule of thumb, until you have revenue and customers you should take the smallest amount possible while not putting yourself in any danger, health wise or other. That holds true until the Founders have found revenue basically.

It is very common to pay yourself minimum wage or so. If you are older and have more responsibilities it is fair to pay somewhat more but just keep with rule #1 above. For a multi-time founder with solid exists it wouldn't be unheard of to see a six figure salary because they have proven their worth. But in most cases you'll see even those prior founders taking the smallest amount possible, it is just that as they gain in personal responsibilities their costs go up so yea they are paid better.

I know it isn't a number, but hopefully that helps a little, it is pretty generic advice because it is also something very specific to the person and situation.