All you can do is look at comps in your area, within X number of blocks and get an idea of what a fair rate is. Although many people will talk about percentage of rent, it's just part of your fixed costs, and the value of your location will determine the value of said rent. You ideally have to look about 3 years in, make your best guesstimate and do a well thought out assessment.
Example: Space A is 1000 sq ft at $1000 per month, and potentially will yield $10,000 per month revenue. Space B is 1000 sq ft at $2200 per month, and potentially will yield $16,000 per month revenue. Which is the better value? Location is not just about amount of traffic, etc. There are a lot of factors that go into choosing the right space. Size. Shape. Lease terms. Surrounding businesses. etc.
Be thoughtful. Don't make an emotional decision on a space you "love". Make a reasoned decision on a space that meets your criteria.