I recently saw a problem on the Inventor Forum, where a user had a bent tube that they had modelled up. They needed to calculate the pre-form length.
In this instance they were not using Tube & Pipe, my solution was to use the Sheet Metal environment to extract the flat shape, the video below explains the process.
There are other things that they could have tried, like simply measuring the length of the sketch, but this does not take material deformation into account. I think that the sheet metal option I used it the best was to go about this, what are your thoughts