I have started tracking my bass growth this year. I found that I want the data at my finger tips at all times.
I have made a spreadsheet in excel that I use through Google documents on my phone. I put in the date, length and weight of the fish, a column then populates telling me the standard 100% weight, and then does the Relative weight % for me.
I find this works great for my needs.
I am working on designing an app to do the same thing, but that is a whole different project.