I thought it would be helpful to write-up how ClimbPro works on the Edge.
This write-up is based on the recent 3.55 software release.
One additional piece of information I would add that is not in the document is that when comparing what ClimbPro has computed with what you experience when actually riding the course will be dependent on the quality of the elevation data that ClimbPro has to work with.
Most of the course planning sites are using DEM (Digital Elevation Models) data to provide elevation data. This can vary in resolution depending on the images and processing they used.
If you have the option to build a course from data collected from some one's real ride using a device with a barometric sensor that will likely give the best match.
I don't really have a way of evaluating the quality of DEM data from various sites. What I would suggest is trying some different sites for courses in your area and see which ones match the best to your real world experience.