when your opening engrams make sure you put your highest light gear on, also it depends on what you do.

this is how it works, if you are a 150 light youll get drops around 140- 200. as you approach 200 light itll transition to you finding something like 190 -240 drops and so on. Eventually you will work your way up to where you can find (the maximum) 260-300 light drops. after 300 light you need to do kings fall raid or events like iron banner to exceed 300 light.
Any other questions just ask

Well it pretty much depends on you're light level, And as Xaldin said, Once you hit 300 light you should absolutely hit up the King's Fall Raid, It's not hard at all if you actually have a good team that knows what their doing. But most legendary engrams will probably give you 280-290 attack and or defense. Pretty much, just keep grinding until you hit light 320!