I consider myself adept with knives, but in saying that, when it comes to knives, your legs, and running, are your best option. Where that is unavailable, in any knife situation I've found that the lack of style, creates far too much randomness to reliably secure a tactical disarming, so I prefer to target the knife myself, and area behind it, namely the wrist. The majority of the blocks you learn, from high block, to into-to-out, and it's respective reverse, do not do much to mitigate the damage of the knife. I find it useful to consider the only part of the knife deadly the blade, when it comes to defense, and by lethal I mean in the same sense one does in aiming a loaded, un-safetied gun. The nice thing about a knife, is that it's only lethal if you're touched, unlike a gun, where wherever it is aimed is respectively lethal, from my pov. Keep one arm in, protecting face and upper vitals, and one arm out, to ward your internals from the blade penetration. Expect to get cut, let them take the arm before your vitals. Turn your body to minimalize impact, and never forget about your kidneys. Those are most vulnerable, in that position, and most likely to be aimed for in a midsection cut. Chances are knife combat will be between you trained, and another with very minimal experience. In all likelihood they are as afraid of their own knife as you should respectfully be. You should always use that against them. JKD has a very important concept for knifefighting, in that when they strike, you react, and get there first. Doing that, I believe strongly, you will maximize survivability.