The best amount is the amount you need, plus a contingency fund. The exact amount depends upon the business and the business environment. How will you calculate the “best amount?” The answer is simple: the business plan.
Business planning is a lot more than just filling in some pre-formatted spreadsheet, or completing the blanks in some downloaded business plan form. If you’ve done your business plan correctly, you will have a tool that will help you model your business under various situations. You will understand the mathematical model that describes how your activities in marketing and sales translate into cash inflow, and what amount of cash you will have as profit. You will understand your USP ( unique sales proposition), your competition, the market dynamics, and how all of the factors: personnel, material, capital and system interact to produce profits.
So what you need to do is work on that business plan. We have a few interesting videos describing the whole process at CEAnow.org.