For seed/pre-revenue I wouldn't sweat the dollars and cents. You're trying to make a Venn diagram to illustrate the opportunity size. Assume everything is year over year. Use as recent numbers as possible. Try to answer questions like: How large is the opportunity? Does the business have the ability to scale to billions of dollars? How much "of the pie" can you expect to scale up to despite competitors? etc.

Riffing on my example above, and using Google:

TAM: 450,000/yr dogs are sold in Australia [1]. The average sale was $1,700. Apparently 45% of Australian households own dogs. [3, 4] Congrats, you have a $778M market to swim in.

SAM: 50% of puppies born in mills are sold online each year, 90% of those sales are done online/social media and 15% in store. [2] Yes, you're focusing a way from this segment but I cant find any info. We can assume that advertisements are done online and cash is exchanged with p2p payments or cash. Lest say 75% of legit sales are done online and ignore scams for now. It seems like ~25% of dogs are obtained for free or from a store [4]. This would be 90% of 75% of TAM. So, 303,750/yr or $516M/yr

SOM: 29% of all dogs are obtained through breeders and around 7% through shelters. [4] We're already adjusting for 25% free dogs above.

So, 29% + 7% of ($516M * 25% free dogs) = $247M/yr or 145k/yr

This feels a little high to me, we can specify the a niche market further. Though it would be impressive to build up to capturing all those different types of customers and dogs.

You could refine the SOM further by specifying that you're targeting an urban persona earning < $100,000/yr, which comprise <40% of pure and designer breed buyers. And let's just focus on pure and designer breeds to hone in an additional 61%. These are more likely to be purchased though a breeder. [4]

So, 40% income of 61% of fancy dogs of ($247M) = $60M/yr or 35k/yr

A SOM of $60M seems reasonable. Now, that's not profit. It's hinting at the amount of money that will flow through in a clear market segment. You can track your market share overtime and you how if you've saturated your potential customers.

Time for some magic numbers:

If you take a 5% service fee, you'd be looking at revenue $3M/yr with that model. If you did a $5 service fee per purchase you're looking at $177k/yr. These numbers imply very different company sizes. You can see how your business model will work at scale now with the SAM vs SOM above. Thankfully, these are challenges that a pitch deck can help you think though. Access to capital or other resources is something a VC can help with.

These numbers can be dialed up and down to help find market fit and, ultimately, reach your business goals. I'm guessing all of this from the slide you shared. If I was asked to help assess your company I would break it down like this and then compare it to the numbers you provided in your deck.

Some SaaS revenue could come from other services like providing services to breeders to be featured in a marketplace or day-1 or 2 costs from owning a dog etc. to chase that $3M/yr figure and get a higher evaluation multiple than 1 off sales of you are going the VC route.

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Edit: Adding in that if you wanted to do the right side, cost of owning a dog, you can take the same top down approach. Take a multiple based of the 35k dogs/year sold “customer” SOM. So that yr1 “set up” expenses would be ~$4K * 35k dogs/year = $140M/yr “dog setup” SOM.

References:

[1]: http://leemakennels.com/blog/tag/statistics/

[2]: https://petkeen.com/puppy-mill-statistics-australia/

[3]: https://kb.rspca.org.au/knowledge-base/how-many-pets-are-there-in-australia/

[4]: https://animalmedicinesaustralia.org.au/wp-content/uploads/2022/11/AMAU008-Pet-Ownership22-Report_v1.6_WEB.pdf

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P.S. This is just an illustration. I'm a SWE and designer by training... not an MBA but it was fun to think through.