Redmine

In the issue screen it is possible to enter the "estimeted hours", for us this is the amount hours we planned before we started with the issue.
The "estimated hours" should be a fixed number.

We don't like the "ratio done" field, we prefer "estimated hours to go". This could result in a ration done of 140% if we still need hours to go but are already over the budget of "estimeted hours".

How could we realise this in RedMine 2.5.0, or is there a plugin that resolves this for us. It is kind of budget control we are looking for.