Hey 
Maybe I can help ...
To do it digitally you still kinda measure things. U can literally apply a traditional method onto digital (Just remember to maintain your posture if you do measurements the traditional way, as any tilts and rotation of your body will affect the outcome).
Another key to success in this exercise is to compare relations and angles of strokes you put down to the ones you can see on the body.
Example; on the male figure above You can see that his right leg is bend. We can roughly see that it creates an angle slightly more than a 90 degrees one. So you draw that. Then you can also pay attention to a negative space between his two legs. It is shaped as a triangle and you check if your triangle matches the one from a reference. And this is how you measure things by "eyeballing" to make them look like the reference.
Just be aware of angles that not only made by an object but also negative space. And the more you train your eye like this the better result you will get, so don't give up 
I hope that I explained somewhat alright and hope it helps ^^'
