I do this as well. I will use all of it out of the last corner up to the start finish line. I also use 100% KERS on the long straight in India. Most of the other circuits i split it up in chucks though.... Like Spa, 40%-50% out of La Source then the rest after Eau Rouge down the Kemmel StraightYeah ...... there is no Optimal. It's strategic most of the time.
For example - If I haven't used it to defend or attack at Interlagos, then I'll save it for the uphill climb towards the start/finish line, turns 14 to 15 onwards