I was calculating the size of ports nescessary for my SBB4 tempest subwoofer I am building. Anyway, I plugged the width of the PVC(3.75 inches according to my measurements)and the desired tuning frequency(18.8hz)into LspCAD Pro. It told me that the required length was 49.4 cm(19.45 inches). Does this sound right? Also, I thought PVC was supposed to be in whole numbers(3 or 4 inch) is it normal for it to measure 3.75 inches? I plan on crossing over to the sub at 100hz. In the frequency response graph it shows a response of 20hz-100hz(+/-3dB) this seems very good to me. Is this an acceptable result? And does LspCAD take into consider the room dimensions I entered in?