better to use calculations based on the characteristics of your DME.
you'll see that while sucrose/dextrose are prety consistent in terms of fermentability, DME may change from brand to brand.
so if you are primming with sucrose, it is easy to give a recomendation based on weight/volume. but with dme is different thing, specially if you are fine tunning the carbonation level acording to style guidelines.
so try to figure out the fermentability of your extract and use the link (calculator)