Make sure you decide what legendary you upgrade. There are better legendary bits found in raids etc giving better defence and light levels than say pieces bought from the vanguard. I had to dismantle a fully upgrade armour piece and I used 18 shards to upgrade it. When I did I got 2 back lol! Use the ascendant materials wisely.

If I recall correctly, it's 11 points over 10 for each level; - 21 Light = Level 21 - 32 Light = Level 22 - 43 Light = Level 23 ... - 109 Light = Level 29 - 120 Light = Level 30

I think if ya press ya options button n get ya guardian on ya screen u can put ya cursor on where the strength,discipline,intellect n so on is there is one that says light if hover on that it will tell how much light u have either got or it mite b how much u need not sure which way round it is hope this helps mate