Hey all, if you're looking to maximise your light level gains from engrams, I've got the method for you!

First, it's important to know that the light level of decrypted engrams is partially determined by your current light level at the time you decrypt the engram. So this method is all about maximizing that value. Towards that end, always keep a higher light level item (even if it's just 1 light higher, and even if you never ever want to use that item). Also, when following this method, you should always decrypt engrams in order from lowest rarity to highest rarity (rare before legendary before sublime, basically).

Step by step:

1) Talk to the crytarch. Do not decrypt anything, just get into chat with him.
2) Now access your inventory (you could also back out, but this is fewer clicks, and I prefer efficiency).
3) Equip your highest light level gear in each slot.
4) Now, comparing slots for which you have engrams, assess which of your pieces of gear has the lowest light value of your equipped gear.
5) Back out of your inventory and you'll find that you're right back to talking to the cryptarch. Efficiency!
6) Decrypt ONE engram for the ONE slot you found had the lowest light level.
7) Open your inventory again and inspect the new item. If its light level is higher than what you have equipped, swap items.
8) Repeat steps 4 through 7 until you are out of engrams.

Cheers,
Kahzgul