There's currently two ways to get the steel dimension for your PrintNC build :
The Fusion360 is considered as the main source of truth, so you should always verify your dimension using it.
After entering all your parameters in the Fusion 360 model, you'll be able to verify the size of all the steel tubes using the Inspect tools. Select the opposing sides of the steel and it will read out the measurement. If the measurements do not match your filled in parameters, go back and check the parameters again.
Here's a video of this procedure.
You can find the official PrintNC calculator on the main PrintNC website.
The metric calculator is available here.
The imperial calculator is available here.
In addition of the obvious difference in the units being used in both calculator, the metric calculator has also another step, where you can send an automatic email to the ThreeDesign store on Aliexpress.
The imperial calculator does not have this step.