I am a bit puzzled about your 180 px. As far as I know Pixels do not exist in Scribus. You should only use the measurement systems that are on offer. Scribus is basically a programm for producing printed matter, however, that can be presented on the sscreens too. So, it is either the metric system of mm, cm, meters, or the typographical of points, ciceros etc., possibly also the imperial- and/or US-inches.
Pixels are a relative size, they depend on your hardware. If you have an old Screen, you have probably a resolution of 60 to 70 Pixels per inch, if you have a modern monitor, this is around 100 ppi. So your 180 px look on an old screen much bigger, than on a newer monitor. On my Monitor, 180 px equals 45.77 mm (17/8th UK inches). Same iI got in Print with my printer.
So, in my opinion, your problem lies more in mixing two things which can not be mixed. Measure our the necessary width you need in mm's, cm's or whatever measurement you use and finds its analoies in Scribus.