The most common mistake we see is a 4x4 tent paired with a light built for an 8x8 room. Overdriving a small canopy does not add yield past a point - it adds heat load, and heat load is what actually caps most home grow rooms, not light output.
We ask three questions before recommending a fixture: canopy footprint, ceiling clearance, and whether you already have exhaust sized for the heat a bigger light would add. If the answer says the smaller unit is enough, we sell you the smaller unit.
Nutrient dosing works the same way: a 3-part line at full strength on a seedling burns tips before it helps anything. We publish a dilution table with every line rather than a single number, because your water hardness changes what "full strength" means.
