The watering level is already being set through the API and works well. This morning the WL was 131% and the sprinkler times increased accordingly.
Thanks for confirming the operation of the system. Alternatively, I can set Rain Delay through the API if rain starts, then zero the delay if the rain stops or rsn Reset All Stations if it reaches 1/3 ETo (for example). There are three NUCs monitoring the weather, thanks to Microsoft Auto Update problems. Each has a time slot and all take turns sending WL commands to OS at 50 seconds after the minute, just before sprinkler programs start.
I noticed that Wunderground updates continue smoothly despite my activities, letting me calibrate Zimmerman parameters as a backup. Very nice product, thanks to all who have contributed!