The protocol you are using will not only leave the sample with rRNA but also non coding RNA.
Many RNA protocols will separate mRNA by affinity of a carrier to the polyA tail. This protocol references an older paper that estimates that only 5% of RNA is mRNA. I'd be surprised if this ratio changed by more than 2-3 fold in drosophila.
I assume that %age is by weight but it could be a densitometry measurement which is similarly interpreted.