Right now the only information the game gives you is:
280 + 290 = ??? - *nail bitting* "what's it going to be?"
It should be:
280 + 290 = 288*! - "Alright, Let's do it!"
When you go to infuse something it should tell you the out come before you confirm it.
*actual light levels may vary. Numbers used for example purposes only. Infusion is based on 80% increases and I'm fine with that. I just don't want to do the math each time. It's not game breaking, it's just quality of life.
*edit* added clarification
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Good idea to have it in game, but as of right now there are tons of reliable websites and apps out there to use. I personally use the app called Infuse Destiny that I got for free from the apple app store. It calculates what the light level will be when using legendary or exotic items and so far its been spot on.