Thanks for sharing the screenshots. I should have mentioned that if the Chrome Device Emulator doesn’t already have your device listed, you would need to manually add it using the effective device resolution.
One thing to keep in mind is that the effective device resolution is different than the physical resolution typically reported in device technical specs as most devices use some sort of scaling. The easiest way to figure this out is to go to https://whatismyresolution.com from your target device and it should show the effective resolution. In the case of the Fire HD 8 it’s 962x601. 
Or you can do it the hard way...
If you prefer doing things the hard way, you can usually look up the resolution and calculate the effective resolution. For example, there’s a table in the Screen Layout and Resolution | Fire Tablets article by Amazon where they report that the Fire HD 8 has a screen resolution of 1280x800 and a scale factor of 1.33.
So to get the effective screen resolution, you would divide the physical resolution by the scale factor. Eg. 1280 / 1.33 = 962 and 800 / 1.33 = 601… giving you an effective screen resolution of 962x601.
So in your custom device profile in Chrome Device Emulator, you would enter something like the following:
Add a Custom Profile
Filled in with Fire HD 8 details
