Do you have a workbook showing your progress so far? I'm rather unclear as to what exactly you're computing RMSE against. A linear regression?
Please attach a packaged workbook, and we can see how your calculation(s) need to be structured to work at different levels of detail.
@Jamieson I have attached a sample package workbook
Thanks for sharing your workbook. The formula you are using is exactly right. The key is to write it as a simple aggregation calculation (which you can tell because Tableau notates it as AGG() when you drop it onto the view). A simple aggregation calculation will correctly compute at whatever level of detail you have on your view — hours, days, whatever.
If you thought that the RMSE was not computing correctly at the DAY level, you might have made the same mistake I did — I assumed that the HOUR level of detail included only one record per hour, and I attempted to pull that output into Excel to double-check the results. When they didn't match up with what Tableau was computing at the DAY level, I got confused. Then I realized that even at the HOUR level, several rows of output are based on multiple rows of underlying data. Once I brought the raw data into Excel, then I could confirm that Tableau was correctly computing RMSE at both the HOUR and DAY level.
So… yeah, in answer to your question, Tableau is correctly computing your RMSE at multiple levels of detail, and you should be good to go!
Let me know if you have any additional questions.