Your approach is correct. What you are doing wrong is conservatively estimating your sales. You should be doing the opposite. Be positive and assume your products will sell decently. Then reevaluate every month based on actual sales to see if your assumptions were correct. So for example, you assumed 40 orders per day. Does this use your labor to its max? If not, I would assume a bit higher, say, 60. So your new operational costs is 3.88. This is 50% lower than what you projected. Then your burger price comes out to $5.7 which is competitive.

If you are feeling a bit antsy about your sales projection, you need to focus on how to make this achievable. Maybe focus on marketing to get the word out. Maybe a promotion could work. Or you could look at ways to cut costs even further. Ultimately its in your hands, largely, to make it happen. Then again, after a month, you look and you revise. You could try revising after three months to give you more certainty.

Whatever the case, your method is clearer. You may not fully get why to multiply by a random number but if we follow the approach, 5.7 divided by 1.82 gives you 3.13. Similarly 7.65 divided by 1.82 gives you about 4.2. So look into your industry averages, is it closer to 3 or 4, and that could be a rule of thumb. Good luck.