Pro Forma Expense Ratio on Properties under 100 units?

I'm right now looking at properties in the 20-60+ unit range. The ones I like to call "tweeners." Some may require onsite property management but the majority do not. I was wondering what kind of expense ratios you guys use for deals this size? I feel like if I use the typical $4-5K/unit I would use on larger properties (100+ units), I would be underwriting myself out of a lot of opportunities. Any rules of thumb you go buy when you're looking at smaller properties on when you think they require staff on site or not? Full time or part time based on unit count if you think it people at all? Any advice is appreciated.