Using the sum function with a blank total
 

I have 3 rows. The first one is time on position, the
second one is total time on, and the 3rd one is the PCT
of time used. I used the cntl-d function to sum the 1st
and the 2nd rows for about 2 months(every day is tallied
even if no training was given). Now my problem is when
one of the days are blank. The program will continue
until the blank, but after, it comes up with "value" on
the day after the no training day. How do I skip the
blank training day total and continue with my sum of the
total training hours(say on the next day.)

Also is there a way to change the color of the font when
the person is reaching a certain percentage (using the IF
function).
Thank you for your time.