Hello, Mrs Jordanova

You describe a very nice way to measure productivity, but you miss some points:

1) Tasks have to be clearly defined ( and explained as you say)
2) Determine how long time programmer will do the task. Say it to programmer.
3) If the task needs some changes, tell the programmer how much time changes will take. ( If pre-evaluation means what I expalin in this point?).Changes have to be good explained also.
4) Clearly say how much you pay for an hour.
5) You can split projects on common tasks, so that you can compare individual projects, and employee performance on task basis

Have a nice day!!!