So apparently destiny figures out infusion by taking the difference of the 2 items, and then multiplies that by 80%.
So if you have a 250 Gun and want to infuse a 280 gun into it you get:
280-250=30. 30*.8= 24. So the end result would be a 274 Gun.
While this is math, it isnt exact. Apparently when its a fraction involved it gets rounded. And apparently if its 5 light level difference it will always get that 5 level's (Again.. mostly hity or miss, but the consensus thus far is hit). Also i've seen reports of people infusing maxed out stuff into other gear and the light level reaching the number of the maxed item. Example (Boots level 275 getting merged with maxed out 285 Boots, and the Boots that were 275 getting all the way up to 285). So the formula is an apparent work in process. But its close.
Take it as you will.
This webpage is the results of someone putting what I typed out.. into a webpage format. Pretty neat.