In terms of setting your businesses Rainy Day Number - anything is better than nothing. But if I was to give any business a target, it is three months worth of expenses to run your business. That means that if you couldn't sell anything for three months, you could pay all of your expenses - this includes paying yourself, profit, tax, and debt, all of that for three months and not have to freak out about it.
This changes your physiology as well, from a scarcity perspective, to a mindset of abundance or security that if things hit the fan, you're in a good place. The key to this is regular - and I would suggest weekly - consistent contributions. As an accountant, a client of Inspire - and we do this for clients with our tax forecaster - we can help you with how much tax to set aside. We can calculate that on a weekly basis if you'd like. It's basic, but it is so powerful.