Anything above 0.6% is good for NP gain

That percentage itself not that important because the number of hits on a units arts card are another driving factor for good NP gain.

So u could say that a unit that has 0.7% NP gain on attack but only 1 hit on a two arts card is not that good.

The general rule of thumb is that any unit with above 0.6% NP gain and 3-4 hits on 2 arts cards have pretty good NP gain on attack.

At least from my experience from playing for a few months.

Hit counts on Arts and Extra, base gain %, attacking a Caster enemy.

As long as it's not negative or zero. We can always buff servants, but if the basic stats is 0 or negative, then it can't be helped.

The number in the stats doesn't matter. What matters is the number multiplied by hit count and any relevant passive skills. Just for reference, not taking any active skills into account, and assuming you're using the card as the first card in the chain, for arts the worst in the game for face cards is a three way tie between emiya assassin, ranmaru x, and the valk assassins at 3.68. (all of their NPs fare much better than the face cards though). The best for arts is ereshkigal at 13.98. For quick the worst is caster Gilgamesh at 0.64. The best for quick is Okita Souji saber at 5.55. So using those values as a scale will tell you how good a servant's NP gain is, although you have to factor in NP gain skills and card type up skills too because those will improve servants you think have might have bad gains.

Edit: for arts you need to do base NP gain * hit count * 3 if it's an NP or 4 if it's the first face card in a chain because the +100% first card bonus does apply to the first card itself. Quick is just 1X of course.