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.