For someone with 20:20 (6:6) vision SD means your screen should fill 20 degrees of your viewing angle, HD 30, and UHD 40. Recommendations for most viewing is 3

For someone with 20:20 (6:6) vision SD means your screen should fill 20 degrees of your viewing angle, HD 30, and UHD 40. Recommendations for most viewing is 30 degrees, but for the fully immersive THX/Theatre style viewing you want 40 degeres. That means for watching your 70" TV you should be about 8' away. If you are watching UHD then that's great, you won't see any resolution artifacts, but with HD you will see a difference. However if you're 9' away from your screen you won't get any uhd resolution benefits. (That's setting aside other UHD features, especially HDR) Of course maybe you have better than 20:20 eyesight.