The present invention is directed to construction tools. The present invention includes tools used for applying mastic or mud to a work surface, for example, a flat box mastic applicator tool, blade assembly, and/or blade adjustment system, that have many improvements too numerous to mention all in the Abstract. The flat box mastic applicator may include unique walls. The blade assembly may have a non-uniform cross section, e.g., a blade holder with thinner areas and/or material removed at various locations along its lateral length. The blade holder may have a larger size outer end(s) to withstand bending or braking. The blade holder may have and arched shape, ribs or bumps for holding a blade more firmly, and a ridge or bump at its outer edge to act as a dam. The blade adjustment system may include symmetrical pins or posts and a tapered leaf spring. See further improvements herein.