In my experiance, and depending on the HDR master, I get more pleasing results using values not quite the same as my measured peak brightness. For HDR->HDR target peak nits should be set to the measured white level, HDR->SDR is less exact. Again, mastering has a huge effect as well, some of my HDR movies need very different target peak nits to look decent in SDR.
And 200 is about right for the brightness of a XB270HU. I have one myself and 160-220 nits is a good brightness setting for it, without a meter simply adjusting it and evaluating the result is all you can do.