About the feature of setting specific amount of water: yes, this can be added, but I am thinking: assuming the flow rate is relatively stable, a fix number of gallons should translate to a fix amount of water time, right? For example, if the flow rate is about 0.2 gallons per minute, 5.2 gallons is equivalent to 26 minutes. Well of course if the flow rate varies significantly this won’t work.
About setting water time more precisely, to units of seconds, this is being addressed in firmware 2.1.7. This is not simply a GUI change, but requires firmware change. I’ve already added the feature, but there are other features of 2.1.7 that I haven’t completed yet so it’s not released yet. Trying to get it done as soon as possible.
In the meantime, there is a work-around, and that is by making use of the manual watering percentage. For example, if you want the granularity to be down to 15 seconds, you can set the watering percentage to 25%, then multiply your desired water time by 4 and set it as the water time. As an example, if you want water time for zones 1, 2, 3 to be 5 minutes 30 seconds, 3 minutes 15 seconds, and 7 minutes 45 seconds, you will need to set the water time to be 22 minutes (5.5 x 4), 13 minutes (3.25 x 4) and 31 minutes (7.75 x 4) respectively, then set 25% watering percentage. That way, when the program starts, the resulting water time is exactly what you wanted. Of course, the downside of this is that you won’t be able to use automatic weather-based watering percentage any more.