I would tend to think that having a general idea about main region population is a good idea. 100m is definitely too low. There are about 7bn people on eath thinking europe only account for 1.5% seems like a serious case of a candidate not having critical thinking and not putting his results and estimates in perspective. In order of magnitude you would have: China population roughly equal to india. Europe is around half of that and the US half of europe. You could also round up europe population to 500m and see if that works for the interviewer.

In any case, you should challenge every single estimate you make and check with the interviewer if your number is accurate. For instance if you need the population of england:

"Given that we are currently 6-7bn people on earth, I would say europe population is probably around 10% so around 600-700m. Germany and france are big countries and are around 60-80m poeple so I would estimate the UK population to be a bit less than that so maybe 50m. Does that work for you?"