I have no personal experience in this, but the area interests me greatly due to the state of flux of my personal life/practice right now.
I have been watching the Joel Sinkin webinars for Accountants World the last few years, they are very informative.
I emailed Joel directly about my situation, but his firm/services are not affordable for small shops. He typically won't work on a deal for less than $10k.
He did write back to me, and his website has a great deal of information available for free that is worth reading.
The 2020 webinar
https://www.youtube.com/watch?v=DMeeAkBalZk2019
https://www.youtube.com/watch?v=96gSV2kqQCs2018
https://www.youtube.com/watch?v=7c87kBEWN6cIt makes sense to me that the retention rate is set based on the 2nd year client list.
The first year you have to assume the client will come out of american lazyness, and if they come back for the 2nd year then they are now your client. They saw the change, accepted it, and are now your client to keep or lose depending on how you service them. A client retention based on the first year only is greatly in the sellers favor, as most people will not make a change until after that initial year, based on articles I have read.
Retention rates can be at or over 80%, but you usually want the seller to stick around to help clients stay, and the seller should want that as it makes the buyer and seller the most amount of money.
Be wary of buying clients that won't be alive to resell in 10 years. You spend 3-5 years paying for them, and then they age out, or no longer need your services.