I count my cards like in Black Jack. If I have more points in my hand than BB´s in my stack, I go for it. easy to decide, cause any card can make it when heads up
It all depends on your starting stack and what kind of position your opponent is in, for example if it's hyper turbo 500 chips then I would be getting in every opportunity to keep the pressure on. However if it's 1500 chip turbo, you can pretty much go too 500 before shoving all in but ideally I would say in the region of 800 upwards.
With a short stack in heads-up with high blinds - you just have to hope for good luck. In all-in you need to go with any pocket cards, the probability of winning which 50% or more.
In those blind depths don't forget that you can add some open limps in your play.It is super rational especially if you feel confident about your post flop game!
15 bb - it’s definitely better to refuse to push. But closer to 7-10 you can play only push-fold. before that it is better to balance between limps and raises (if the opponent allows it)