Abstract: A device for determining a precise placement of a medical sensor on a patient's chest is provided, the device comprising: a vertical portion including a sternal notice index portion for alignment with the patient's sternal notch, wherein the sternal notch index portion extends along a portion of the patient's sternum, wherein the vertical portion includes vertical patient size indices; and an arm extending from the vertical portion across the patient's midclavicular line, wherein the arm is oriented at a right angle to the vertical portion, wherein the arm includes horizontal patient size indices.