I've added some more features to the little scale speed calculator I've knocked together. It now calculates the average time and speed for all the runs combined, and to stop erroneous runs being recorded there's a cancel button that can be used after a run has been started.

http://www.entwinesolutions.co.uk/scalespeed

One tip I should pass on is that after you've clicked on the start button for the first time you can just use the enter key to start and stop runs. This will continue until you click on something else like the cancel button. To be able to use the return key to start and stop the tool again just click on the start button with the mouse again and then you can go back to using the enter key.