Ok, so you've already got version 1 of your product in Excel. So, my first question is: have you shared that with any of your audience?
Why do you want to make it a web app? Why don't you just sell the Excel workbook itself. Make no mistake - it's a big readymade market. See this for inspiration - http://startupdesi.com/100000-startup-revenue-in-12months/.
It might at least be a good starting point to get hold of your customer base.
Presumably, you can create a much more useful version if it's a webapp.
So then, the question is how do you market it to your customers?
It's great that you yourself are a user of the app. You exactly understand the pain points of your users.
Now, where do these people go to practice their shooting (or whatever the technical lingo is)?
Are there clubs? Are you a member of any of them? Can you make a presentation in any of those places?
Are there relevant magazines? Newsletters?
Have a look at Tim Ferriss' 4-hour work week for ideas on advertising in the right places.