Translate DPI setting from old to new mouse

I had a Razer Deathadder 2013, which apparently runs @ max 6400 DPI. I bought a new Razer Deathadder v3 PRO, which can do between 100 and 30k. I set it up at 6400 but it is way more sensitive. I actually get a more exact match at 1200 DPI. How can I get the necessary info to calculate/get the DPI I should set on the new mouse? It is possible that my previous mouse was not at 6400, or that other settings my interfer. I'm testing on Windows, not on a game or anything so far.

Question by Bob Tajid on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.