Your statement is essentially correct, but over-simplified.

VBR gives you "more bang for your buck" in terms of file size. In other words, if you had two 5-megabyte files of typical music, one of them a constant-bit-rate file and another one a variable-bit-rate file, the VBR file will generally have better quality. Because it can crank up the bitrate for complex (high-frequency) passages, and reduce the bit rate for less complex passages.

I don't know about "percentage" settings because my VBR encoder software doesn't use a "percentage". With VBR, the resulting bit rate and file size are kind of a crap-shoot, you never know exactly what you're going to get. You just choose a setting and try it out for a while and see what kind of file sizes you're getting.