Parker Smith,
I too was very frustrated that Apple Engineers changed the way the macOS calculator in Notifications works now. Many people state that it is "broken", but people need to understand the difference between software being coded with an error ("broken"), and intentional programming to function as directed. The macOS calc is not broken, it was altered to function resetting the total to "0" whenever you close and reopen the Notifications window or switch to another program, then go back to it. Unfortunately, they did overlook that those upgrading from Sierra to High Sierra would have the issue of the .plist not being deleted, therefore causing the total in the calc to repopulate to the last total that was present in Sierra.
Many in this thread are mentioning the "solution" mon.fractus gave, and although it does delete the .plist file so that the calc now resets to "0" (as Apple Engineers programmed it to do now), it isn't the solution that the majority wanted. We wanted it to continue functioning as it did in Sierra where the last total given would always be present, unless we cleared the calc. But, as I stated after receiving the reply the Engineers forwarded, this is now the "normal" function of the calc in Notifications, and not a "bug".
Lastly, many also keep suggesting to use PCalc which does appear as a widget within the Notifications side window, but I don't really want to spend $10 on a calculator app/widget. Also, it has way too many scientific functions that I personally do not use. That's why I gave the suggestion of using the free app/widget PEMDAS. I don't see the issue of setting it up in Dashboard as an overlay and choosing a Hotkey for it. It actually is better, at least for me, than having it in the Notifications sidebar which would close the moment you go to any other window. My suggestion at least allows you to see your current window underneath as you enter your calculations.
In closing, the macOS High Sierra calc in Notifications will no longer function as it did in macOS Sierra. You can spend $10 (PCalc) to have a calculator in Notifications that keeps the previous total, or you can install a free app/widget like PEMDAS (or similar) and set it up as an overlay in Dashboard as I explained before.
In God's Harmony