I tend to assume 1 Avios = 1p which is near enough to the rolling average.
I will cost promos on this basis and sometimes won't bother as a result, but I rarely if ever let Avios earning influence my flight purchases.
Likewise on redemptions, always happy to save more than a penny per but it's the convenience of buying a (practically) fully flexible ticket (GGL so no fees) that really attracts me
