40 year projections are about as worthless as a used Barry Manilow 8 track.
GDP growth is the standard litmus test of how an economy is performing. National bodies such as the Office for National Statistics calculates it each quarter by assessing the activity in various parts of the economy. They then attempt to strip out the effects of inflation by recalculating the activity using last year's prices (this is called the 'chained volume measure', as it links the two years together to give a better picture).
GDP also ignores a lot of personal activity - hire a company to repair your house and this activity will be measured by the ONS, but do it yourself and it won't be. Additionally, it doesn't measure discrepancies within an economy, so GDP-per-capita does not distinguish between societies where there is a large gap between rich and poor, and a small one.