Concrete block is designed around a 16 inch module including the mortar joint, which gives 1.125 blocks per square foot of wall for standard 8 by 16 units. That figure holds regardless of block thickness.
Design to the module
A wall laid out in multiples of 16 inches horizontally and 8 inches vertically needs almost no cutting. A wall designed to an arbitrary dimension needs a cut block in every course.
Openings sized to the module save the same effort. It is worth adjusting a plan by a couple of inches to land on it.
Reinforcement and core fill
Structural block walls have vertical rebar in grouted cores at spacings set by the engineering, plus horizontal joint reinforcement every few courses.
Grout for filled cores is a separate volume calculation — roughly 0.19 cubic feet per core for standard 8 inch block. Whether and where to fill is a structural decision, not an aesthetic one.
The footing decides everything
Block walls need a level, properly sized concrete footing below frost depth. Everything above copies the footing, and a footing that is out of level cannot be corrected in the first course without a very thick bed joint.
Snap lines on the footing and dry-lay the first course before mixing any mortar.
Getting the first course right
The first course of block is laid in a full mortar bed on the footing, and it is levelled in both directions and squared before anything goes on top of it.
Every error in that course is repeated and amplified up the wall. Time spent on it is not wasted, and professional masons spend a disproportionate share of the day on the first course for exactly that reason.
Dry-lay the first course before mortaring to check that the wall length works out to whole blocks, and adjust the layout before committing.
Parging and waterproofing below grade
A block foundation wall below grade needs a coating on the outside — parging with a cement mortar, then a waterproofing membrane or coating over that.
Block is porous and the mortar joints more so. Backfilled straight against bare block, a basement wall will admit water through the joints.
The drainage at the footing is the other half. A perforated pipe in stone at the base of the wall, sloped to daylight or a sump, is what keeps water from standing against the wall in the first place.