If I have as much business as I can handle then I'm at capacity. If not, then I need more business so I market more. It doesn't matter how much money I made in the month before, either I have as much as I can handle or not. Thus, some months I spend more than 100% of my income as marketing, other months much less.
Just picking an arbitrary number like "10% back into marketing" is like throwing a dart at a moving target without even kowing what you're trying to hit.