@firefly: since a few firmware versions ago, we’ve changed the way master zone is implemented, as a result, the master off delay can only be negative, and master on delay can only be positive. However, there is a work-around you can use to achieve the effect of positive master-off, that is, by creating a dummy zone (a zone that’s enabled in software but not a physically connected zone). For example, if you have 16 zones, you can designate zone 17 as a dummy zone. Make sure that zone activates the master (by default it does), and include that zone in your program, for how long you want the master to remain on after the last zone closes. This way, at the end of the program, it will run the dummy zone but since it’s not a physically connected zone it won’t actually turn on any sprinkler, other than keeping the master on for the duration you programmed the dummy zone for.
The latest firmware supports up to 72 zones, so as long as you have not exhausted all 72 zones, you can always designate a dummy zone for this purpose.