Your calculation seems to be correct.
About the second question. You need to define the "trascriptional units". if your aim is to establish a "gene level" expression your trascriptional unit should be the full gene exons set. If you are interested in "isoforms level expression" you should calculate rpkm for each isoform.
here you can find some examples:http://woldlab.caltech.edu/rnaseq/