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 20 and 15'' MacBook Pro (2015 MagSafe2 model) for example, Omni 20 has 73Wh capacity; 15'' rMBP has a 99.5Wh internal battery; both the Omni 20's conversion efficiency and 15'' rMBP's power supply efficiency are around 90%. Put these numbers in the above equation and we get:
73Wh / 99.5Wh * 90% * 90% = 0.59
This means that Omni 20 can charge up 15'' rMBP for around 59%. Of course, the 15'' 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.
Something about DC output:
One of the advantages of Omni20's DC output is that you don't need to suffer from the laptop's power supply efficiency loss. And the above equation becomes:
Omnicharge's battery capacity / laptop's internal battery capacity * Omnicharge's conversion circuits efficiency
Again take Omni20 and 15'' MacBook Pro for example:
73Wh / 99.5Wh * 90% = 0.66
This means that Omni20 can charge up 15'' rMBP for around 66%. That's 6% more compared to charging from the AC outlet!