Pre-planned artillery-- when to call it in?

[Bert Blitzkrieg]

If I can I buy some conscript FO's so to get the biggest bang as cheap as possible and then target an area which I want to pass/take. Try to estimate at what turn i will show up there and then pre-plan my arty strike(s) a few turns less.
I like to mix medium arty (like 105mm) with the real heavy stuff. The medium arty will prevent a retreat and the heavy stuff is meant for the big kill. If there are enough points of course. Otherwise I only use medium arty.

The light arty I try to use as an opportunity/reason arises (smoke or guns etc) and be flexible with it. it is to light anyway to really soften things up.