Scale of material when using Cubic UV's

I'd like to accurately scale my materials when using Cubic UV's instead of guessing as to when it looks right. What does TM assume the scale of an image map to be when first brought in? For example, I am using the attached tile image which represents a 2" x 8" tile which would be 64" square in real-world measurements. If I use it at full scale (1.00), what does TM assume is the size of the overall image (in imperial or metric units) when first placed at full scale? If I knew this I could change the slider accurately to reflect the real-world size. Further, it seems counterintuitive that once the materials is assigned using Cubic UV's that the UV's would stick and remain in the same proportions when scaling the actual object. Please advise on the assumed size when an image map is brought into TM using Cubic UV's. Thanks!

tile_2_x_8_vertical_modern_SMITH_BaseColor_1