I've been trawling the internet to see how infusion works, specifically around the maths part. This is how it seems to work for people:
Identify the kit you want to infuse (increase its light level).
You then need kit in the same slot (special weapon if you want to upgrade a shotgun for example) that has a higher light level. This can be blue, purple or yellow. Only purples and yellows can be infused into though
When you press the infuse button on the kit you want to upgrade the game identifies potential kit you can dismantle. So you don't use the resources until you know exactly what you are going to dismantle, so don't worry about just checking it out
How much upgrade you get seems to work like this
You get 80% of the difference between existing light level and the light level of the stuff you dismantle. I'm not sure how it deals with fractions, it might round up, down, to the nearest whole, or might not round at all (just might not show the decimal point)
Example - if you have a 280 legendary and a 288 blue, you can infuse the blue into the legendary and expect to get a boost of 8*80%, which is 6.4. I think this rounds down to 6, but its hard to confirm. You will end up at 286 or 287 though (maybe 286.4 showing as 286). You wont get to 288, which is why its a big decision to give up that 288 blue.
Cant find anything on how it treats the decimal point, but the 80% seems fairly well confirmed on t'web
Anyone else finding anymore out please add to this