This question is rather complicated since your laptop's consuming power fluctuates a lot depending on if you're gaming or simply browsing the Internet on the laptop. This apparently, affects the time and percentage Omnicharge can charge your laptop.
So to simplify the question, we use a more objective method by comparing Omnicharge's battery capacity to the laptop's internal battery capacity. Also considering the efficiency loss of both Omnicharge's conversion circuits and the laptop's power adaptor, the equation of charge percentage is:
Omnicharge's battery capacity / laptop's internal battery capacity * Omnicharge's conversion circuits efficiency * laptop's power supply efficiency
Take Omni 13 and 13'' MacBook Pro (2015 MagSafe2 model) for example, Omni 13 has 49Wh capacity; 13'' rMBP has a 74.9Wh internal battery; both the Omni 13's conversion efficiency and 13'' rMBP's power supply efficiency are around 90%. Put these numbers in the above equation and we get:
49Wh / 74.9Wh * 90% * 90% = 0.53
This means that Omni 13 can charge up 13'' rMBP for around 53%. Of course, the 13'' rMBP should be powered off or in sleep mode since operating the laptop while charging it at the same time can shorten the charge time and percentage.