@Maximilian Marck sorry this hasn't gotten much traction. Hopefully getting a bit of conversation going inside the post can get that going. From a generalized PoV I would say your 20% will be appointments (with sellers set), appointments attended, offers made (&accepted).
To break it down a bit further, KPIs would be how many (insert lead generation tool here. Example: mailers) result in 1 lead, then how many leads result in 1 direct conversation with a seller, further on to how many conversations do you need to have to set an appointment, how many appointments attended to 1 offer, then how many offers made to an accepted offer.
In the end, the business is not unlike others in that it is a game of marketing, and then selling your product. So, to find your 20% EVERYTHING should be measured, whether that is in an excel sheet you keep updated, your CRM, or somewhere else. Unless you run your business like a business, it will be really difficult to truly determine a good 80/20 rule