Good points countrigal .... another good time is to take your lunch after or before everyone else.  Eg. if most people go for lunch between 1-2pm then you should take yours between 12-1pm.  When you come back to your desk everyone else will have gone for lunch and phones and office will be quieter.

Countrigal is right though - managers always want PAs around them.  The fact that you will be working flexi hours means they will panic about you not being there to call upon.  I suggest you insert in your procedures the continued need to sit down with managers etc and address issues eg. diary commitments, post, email etc, etc.  If they know what hours you are available, they might be more willing to work round you.

I know lots of PAs and admin staff who work flexi times and as it's company policy, there seems to be no hassles.  Also lots of PAs are busy mums so again employers are expected to offer flexible working hours to benefit and keep their staff.

Good luck!