As I design on open source software and one of my favorites is Inkscape but by default it operates at 90 dpi and not 72 dpi like the web. I was having real problems exporting my web design mock ups at the correct sizes. I worked out a simple solution to this by dividing 90 by 72 giving 1.25.
the significance is that to export at 72 dpi with the correct proportions simply scale your design by 125 %
Or, you could just change the default export resolution in Inkscape
File->Inkscape preferences->Import/Export->Default export resolution.
As i normally print out my designs, mine gets set to 300 dpi most of the time…
hope that helps / saves you from doing any calculations
Even when you change the default export resolution you are still working at 90 dpi within inkscape. If you have a box with a width of 700px unless you increase the size by 125% you will export the box at 563px with the dpi of 72. oh and dont forget to take into consideration the stroke/lines.
also you don’t get the same problems when upscaling the resolution to 300 dpi
This would only pose a problem if you prepare your mockups for print. Some people think that 72 dpi is a must for screen display, but DPI values have no impact there at all, only pixels matter. An image with a certain pixel count will display the same on a screen whether DPI is set at 1 or at 1000. So for web purposes it suffices to export the image at correct pixel dimensions and ignore the DPI.
Of course if you want to export the images persay at 72 dpi your method works, but as said before it only affects print quality.
By the way the basis for the 90 dpi in Inkscape is set by the CSS reference pixel.