From there, it gets trickier, but if you've built a good product that relieves some need without causing too much additional pain, you'll (possibly/probably) be fine.
Shouldn't data exist in the form of direct competitors? If there aren't any, then does this market exist? (It's entirely possible that it does, and no one but you sees it yet).
Data should also exist in the form of knowing roughly how many people could benefit from your product (and who might consider it a step up). If you're basing this on a market need, then that population should be at least a somewhat knowable quantity.
So, basically my angel's roi is not clear and a convertible note with a reasonable discount sounds ideal but the system here is the system and I don't want to waste time fighting it. I will put in a business plan to get talking to the right guys and then try and convince the right angel to ignore it and do a convertible note and hopefully all will be well in the world again. This will have the unfortunate side effect of polluting my mind with fake figures and expectations which I wish I could avoid but that's just part of the game at the moment.
Either roll up your sleeves and bootstrap the beta or have professional courtesy to sit and honestly write down, in-depth why you think it would be profitable for someone to give you money.