I'm nervous about this change because I don't understand it.  I had tuned the calc expression to give an integer value on a device with 320px width.  That caused gave bad borders on your device.  Your device is 480 CSS pixels wide or 720 device pixels, right?  Those are both divisible by 3, so my calc() expression gives a non-integer width on your device.

So if you change the calc expression so that it gives an integer width on your device and a non-integer width on 320px devices, I don't completely understand why that doesn't affect the  borders on 320px devices. I guess that the 105.66 width rounds up to 106 on my device.  And I also guess that the uneven borders on your device are related somehow to the 1.5 device pixel ratio.

Since I don't understand this I'm not convinced that there won't be some other future device that has bad borders with this width setting.  Its okay with me if you land it like this, but I wonder if in the future we will want to make thumnail width configurable on a per-device basis. Either with a vendor-configurable setting or by using media queries to set the thumbnail size explicitly rather than using a calc().