The basic principle is for calculating the Projected life in the performance Combined Reports are as follows: 


Total Run Mileage of the Tyre + [Wear Rate (Actual Mileage Run / Tread Used) * (Avg. RTD - Pull Point (set for the Vehicle) ] = Projected Mileage


The very same calculation applies when recording hours.


Note:

It will not look at the total history of the Wear Rate. Toolbox will only factor in the Wear Rate taken from the latest inspection. 


 A lot of processing occurs behind the surface in Toolbox WEB, hope the above helps you to put some light on this issue.