To calculate the relative scale of a Print Area, you will need your 2D View of the Product (open in your selected artwork package) the physical product dimensions and the print dimensions in mm.
Important Note: In most cases, it is possible to position and scale the Print Areas on a Product 'by eye' without having any implications on user expectation / experience. As the creator of the 2D Product you can follow either method as you deem necessary.
There are 4 steps required in this process:
Step 1 - Obtain the height (or width) pixel value of the Product from the 2D View (800 x 800px).
Step 2 - Divide the height pixel value, by the physical product height in mm to obtain a pixel value per mm.
Step 3 - Multiply the 1mm pixel value by the print dimension height (mm) to obtain the optimum Surface Area height value in pixels.
Step 4 - Change the Surface Area width pixel value to match the Aspect Ratio of the of the Print Dimensions.
Lets cover this process on a 2D Product. In this example we will be using a Phone Case with the following information:
Product Dimensions = w67mm x h138.1mm
Print Dimensions = w50mm x h50mm
Step 1 - Height of the Product in the 2D View = 730px (see figure 1)
Step 2 - 730 / 138.1 = 5.286px per mm
Step 3 - 5.286 x 50 = 264.301px
Step 4 - Change the Surface Area width value to match the AR of the Print Area Dimensions (see figure 2).
We now have a Print Area that is scaled Relatively to the size of the 2D Product (see Figure 3).
More information on pixel to mm conversion and vice-versa is available at this link:
mm = (pixels * 25.4) / dpi
pixels = (mm * dpi) / 25.4