THE ACCURACY IS %98.7

My mobile's ideal width is 7.6/0.987= 7.7mm

*Step (2): After I choose 3mm wall thickness I can calculate the ideal dimensions of the box:

1) Ideal Length= (67.1+(3x2))/ 0.987= 74mm

2) Ideal Thickness= 12.6+(7.7-6.6)= 13.7mm

***Note: 6.6mm is the inside width after 3mm wall thickness has been chosen.