Yes, absolutely, fill out the form with your worst day in mind. It is an awful thing to do, I found it massively stressful. Have you got the Cerebra guide, it is brilliant and you shouldn't fill out the form without it!
I'm starting to gather information to fill in a DLA form for DS who has ASD. Some of his needs 'come and go', for example, he has had a period where he destroyed our sofa by pulling at a hole in the fabric, bit by bit. He then stopped it for a few weeks but started again- and very often the pattern is like this with many of his behaviours. It's really hard to say his needs take us x time per day/week because it seems very cyclical. Is it ok to include activities which don't happen as regulary as every week?