I agree with Richard, but might approach it a bit more specifically:

1. How many frames can you reasonably build in a year?
2. What price can you command for them, being realistic about your quality (quality of manufacturing, finish, and marketing)?
3. Can you sell all the frames you build -- how many purchased in advance (i.e., custom) and how many spec (for sale after finishing)?
4. How many frames will you sell as complete bikes?
5. What is your margin on custom frames, spec frames, complete bikes? Margin at, say, $75/hour for every hour you even look at a lathe or a torch or work on paperwork or shipping.
6. Do you have the capital (dollars) to support the costs of equipment, contracted paint jobs, frame components, equipment groups (for complete bikes), a few complete bikes for sponsorship and display and for test rides, plus advertising, shipping materials, and so on? If you don't have a dollar value next to each of these costs and don't know how you'll deal with the total at the bottom, you're in the wrong business.
7. Do you have the people resources to support what you're doing? Working alone is a very lonely business. You need motivation, support, someone to help do the books, someone to pack up frames while you're rushing to get some others done, someone to track payments and run to the bank, catch your mistakes, brainstorm crazy ideas, someone to stop at Chipotle when you're still brazing at 10 pm. (I work with a group that helps recently discharged veterans start successful businesses -- and my first piece of advice is always to find someone to do it with.).

Business is about hard realism. If you don't bring in the cash to cover your costs, you won't continue to be a frame builder. And you won't enjoy what you're doing. Those numbers for volume, sales, and cash flow are as important as the angles you build to. These threads tend to get general or theoretical or speculative or hopeful. None of that works when you have to make your frame building business succeed. You have to sell a spectacular frame to succeed, but you have to sell x frames a month at y dollars a frame at z dollars fully loaded cost per frame and the trends for all your numbers have to be pointed upwards. If not, you'll be welding lawn furniture next year.