Sorry I had to re-orientate myself with this one lol. It's multaplicative. For example, if you have a 10% bonus in the skill, it increases the recyclers base efficiency by 10% of that value. For example: Recycler with 50% efficiency, and you have 10% on your skill. This would increase the 50% efficiency by 10%, which would be a total of 55%. The percentage is applied to the recyclers number. If you wanted to bring a recycler from 50% to 100% efficiency, you would need to add a 100% total buff value to the skill (0.2 per level).