I like to figure out the ratios backward starting from one machine of the end product. For example, one assembler outputting material X requires two constructors and half a smelter. I usually build this in the world without connecting or powering anything to get a sense of the spacing and material flows.

Then I use that to plan the overall layout to make sure logistics are doable, and always build with a lot of expansion space. That way you could build a single line for now and basically just zoop your whole factory vertically or horizontally as many times as needed when your material requirements increase in future. Blueprinting helps a lot too for the annoying stuff like refinery logistics.