When I see pitch decks that talk about market opportunity, the biggest problem I see is not the "TAM is $X / Y% capture", but rather that the TAM that's being laid out is not actually the TAM for the product.
For example, I've seen a pitch for a product for electricians that used a TAM of the total amount spent by consumers on electrical work. That's not the TAM for your product for electricians.
I've seen a pitch on a commercial real estate startup that managed RFPs use a TAM of "Commercial Real Estate is a $15 Billion asset class". That's not the TAM on RFP management.
I think that if you focus on fixing your calculation of the market opportunity to something that is real and rigorous (and probably has a formula with supporting data that you are also providing), you'll find that the practice of cutting it down by a percentage capture rate is much less problematic.
I disagree with the advice of using Atlassian, because you are a start up. You are not Atlassian. Atlassian can go after crazy large markets like "Application Development" and "IT Operations" because it has a huge capture rate in segments of those markets. As a startup, you don't have anything--you have to walk before you run.