[Grail]

I've never used "percent complete" - it's a meaningless number that ends up getting demoralising as you get to 80% in an hour, then 90% the next hour, then you realise you're barking up the wrong tree and "percent complete" drops back to 20%.

The GTD-ish way IMHO would be: at the end of your session (you're heading off to lunch) write a task to write the next unit test, or write the code to implement the function that your last unit test introduced. Or even "Finish module X"...

Keep your mind focussed on what you need to do next, not how much you think you have left to go.