Here is a similar thread posted a while ago you can check out:
http://www.subsim.com/phpBB/viewtopic.php?t=41143
There are many methods you can use to plot the target speed manually using map updates and time. Chart plotting / distance time as you say is probably the most realistic and easiest. Many of the TDC tutorials you can find on Subsim and elsewhere go over different methods. I can send you the spread sheet I use (see link above) if you want. If someone would like to post it to a website that is fine also.