A settable composition may be designed to thicken at substantially the same time downhole and/or set at substantially the same time downhole independent of when the settable composition was introduced into the wellbore by varying the concentration of set retarders and/or set accelerators during placement of settable compositions. For example, a method may include introducing a settable composition into a wellbore, the settable composition having an initial portion with a first concentration of a set-time additive, a middle portion with a second concentration of a set-time additive, and a final portion with a third concentration of a set-time additive; choosing the first and the third concentrations of the set-time additive so that the initial portion has a longer set-time than the final portion; and thickening the settable, wherein the initial and final portions thicken to exceed a consistency of about 70 Bc within about 3 hours of each other.