I display every 10 minutes the BIG-scroll-ISO.dsp template. However the daily rain shown is not the same as the value from rain0total. In this case rain0total is 7.2 but BIG-scroll-ISO.dsp shows 5.4 . I have the latest build 13361.
The referenced display script shows [rain0total-sumday=mm.1:--] as todays rain. Which variable string does show you an alternate value? "rain0total" is just a sensor, which can be used with different selectors. "sumday" or "daysum" is the right selector to use for your purpose.
7.2 is the value is see in 'Historical data' row rain total in the column Today. And is also the value I get by using rain0total-daysum for my HTTP request.
Then either the display is not updated or you are looking onto the wrong display page (rain of last hour and of today are displayed, when I remember correctly).