Setting aside for a moment the thought of a custom Behmor Interface, this behavior already exists. It sounds as though you are not seeing this - is that the case?
When you choose a valid combination in Roastmaster, e.g. P4, D and 1, you should be presented with the heat curve the Behmor uses. It will display in the console graph.
Then (for example), at first crack, you would press P3 on the Behmor to drop to 50% power. At that point, you would tap the digital readout in Roastmaster, and enter “50”. The curve will enter “Automatic” mode, create a node at 50, and clear out any remaining nodes to the end of the roast.
If, at any point, you wanted to revert back to “Automatic”, you could choose “Convert to Automatic” by tapping the curve button in Roastmaster (next to the digital temp readout). Roastmaster would repopulate upcoming nodes with what they should be on the Behmor itself.
If you are not seeing this behavior, then let me know - perhaps your Behmor roaster definition is not configured correctly, or the preference setting for “Automatic Behmor Curves” is not on.