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.
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.