Anytime you weld a seam, the weld and adjacent area which is called the HAZ (Heat Affected Zone, the area that has turned blue/discolored) is going to shrink as the weld cools. So as an example, in looking at a quarter panel side profile (red line) :
...if we were to weld a vertical seam top to bottom, as the weld cools and shrinks, it gathers up/becomes shorter in length (arrows) along the entire length of the weld. So in essence, the linear length of the weld gets shorter; it becomes a shallower arc, or towards a straight line.. (blue line) This is why ALL welds in a crowned panel, absent any planishing, will pull into a valley. The arc is becoming a straight(er) line. You combat this issue by planishing using a hammer and dolly. You want to stretch the weld to regain the linear length it was prior to welding.
For "sizing" your dolly for the concave back side, you want to match the inside of the crown as closely as possible without the corners touching. (to prevent marking/coining the metal). The convex outside of the panel you would want to use as flat a body hammer, or a slapper, etc as you have. Hold a slight upward pressure on the dolly as you hammer to coax it to move outward.