Well, if you can position the controls in relation to the content, then you also position the content field for the controls, right?
, ControlPlacement , Pane, ( ImageSize, , ).
VMax = 12;
Manipulate[
Pane[ToString[(10^Rx/(10^R3 + 10^Rx) - 10^R2/(10^R1 + 10^R2))*Vin] <>
"V", ImageSize -> {500, 20}], {{R1, 5}, 1, 6, 0.01},
Pane["R1 = " Dynamic[Round[10^R1] "\[CapitalOmega]"],
ImageMargins -> {{2.5, 0}, {3, 0}}], {{R2, 5}, 1, 6, 0.01},
Pane["R2 = " Dynamic[Round[10^R2] "\[CapitalOmega]"],
ImageMargins -> {{2.5, 0}, {3, 0}}], {{R3, 5}, 1, 6, 0.01},
Pane["R3 = " Dynamic[Round[10^R3] "\[CapitalOmega]"],
ImageMargins -> {{2.5, 0}, {3, 0}}], {{Rx, 5}, 1, 6, 0.01},
Pane["Rx = " Dynamic[Round[10^Rx] "\[CapitalOmega]"],
ImageMargins -> {{2.5, 0}, {3, 0}}], {{Vin, 2.5}, 0, VMax,
Appearance -> "Open"}, ControlPlacement -> Bottom]

EDIT [CapitalOmega] \[CapitalOmega]. , .
. Wizard.