Determining image sizes for multiple android screen sizes/densities
-ldpi is ~120dpi, -mdpi is ~160dpi, -hdpi is ~240dpi.
So, a 320px wide image is 2" in -mdpi. If you want a 2" image on -ldpi, you want it to be 240px (=75% of original). If you want a 2" picture on -hdpi, you need 480px (=150% of original).
Hence, your analysis is accurate.