[idvusers] Precipitation accumulation

I'm having trouble accumulating precipitation properly (using the NAM from
the Motherlode THREDDS). I'm using the Formula > Grids > Time Steps
functions, and getting weird results. I assume this is because the NAM
accumulates 3-hourly precipitation, but doesn't "reset" until every 12th
forecast hour... so the accumulation function is adding up everything in
between and giving values way too high. I tried selecting each individual
12th forecast hour, but that doesn't seem to work for some reason.

Any ideas?

Best Regards,

Robert Dewey