You need to include 'qcov' in the -outfmt argument. Here is an example:

-outfmt '10 qseqid sseqid stitle pident evalue qcovs'

Took me a while to figure this out too!

Looking at the blast help for the tabular output it seems what you want is not implemented in blast. Only percentage values available are: "Percentage of identical matches" and "Percentage of positive-scoring matches". You will probably have to compute the value yourself.