We probably have different images of the spellcasting process, no big deal. I tend to use parsimony when interpreting Earthdawn rules, and saying cover applies if you can use a nearby object "for protection" is the simplest way to go. I also tend to imagine the spellcasting process like this:
The magician begins to gather the Mana (free floating astral energies) needed to craft the spell pattern (simple action for 0 thread spells, otherwise Threadweaving). Then, the magician - through force of will (the Spellcasting roll) - "ionizes" a channel of astral space between him/herself and the target. The spell-pattern flies down this ionized channel dumping Mana (astral energy) into the physical plane as the spell effect.
A good analogy is the Navy's lightning gun:
http://www.gizmag.com/laser-induced-pla ... nel/23117/This "ionization" path does want to take the path of least resistance (a straight line), but (in my Barsaive) the magician can force it to arc around cover objects at +2 target's Mystic defense. While the Astral Sense spell does allow the caster to know exact positioning within it's radius for any astrally sensed target, I still consider that your are drawing the arc from your pattern to the target pattern, so the difficulty of forcing your astral path to arc over cover gives the same +2 to target's Mystic Defense. The difference being that full-cover is now only partial cover for astrally sensed targets.
The best thing to keep in mind is that the Earthdawn metaphysics are tied so well to the rules that different rule interpretations can have effects on how your group interprets in-game metaphysics. I tend to start with the simplest interpretation of the rules first, as I know they are already tied to the metaphysics. Keep it consistent and it will be fine.
Still interested in the procedure Dougansf asked about...