This is a very quick way to calculate your flow ratio for new filament. It takes about 5 minutes to print two pieces at normal speed. The Print Profile includes the following settings: 0% infill and 1.2mm shell thickness (3 shells) for 0.4mm nozzle. Wait for them to cool down before removing them as you don't want to flex them when removing from build pate. If you do not own calipers, you can quickly gauge current accuracy of your flow rate by connecting both pieces together and if your flow rate is close, they should fit together loosely so that connecting one piece to the other is snug but not tight. If it's a tight fit, there is some work to do (keep reading).

Once you've removed them from the build plate, use the Google Sheet (here) and follow the instructions. Make sure you input your starting flow rate for your specific filament if you did not already set it to 1.0. Use the calipers and perform at least 3 measurements on each of the pieces - don't forget to flip them over to measure as well. Use this information to calculate your new flow rate. That's it - no need to adjust your diameter or density - just adjust the flow rate and you're done!

Original credit goes to nirdobovizki for the original work.