Technically it doesn't matter. Every IFG block has exactly 10 times the coverage in a given dimension relative to its own size in that dimension, effectively 9 times if the IFG block is excluded, from its center to the center of the farthest block it can cover. How would you divide it is largely irrelevant. You're good as long as fields generated do not intersect each other. Two nearby blocks with fields directly adjacent to one another are just as effective as one block of equal volume in the middle. If you're very meticulous about it, you can reduce the amount of IFG blocks even below 1% of Processing Power, but it is way too much bother that it worth.
Theoretically, you can get way below that. Since a block benefits from the full effect of an IF when it only touches it, you could cover all your blocks while only covering a tiny fraction of the actual volume of the ship with IFs. For example, a 0.001x0.001x0.001 IFG at the intersection of four 10x10x2.5 armour panels only takes up 0.0000000001% of the processing power.
Personally, I don't bother going that far. I usually place several IFGs around the ship's surface under the armour layer, but I just do that by replacing the same kind of smallish framework blocks that I also use for other components when the shape is roughly finished.