voltage divider and running other circuits causes different voltage readings

Using Arduino Project Guidance
3

My maths isn't the best, I've been using this as a reference - http://cdn.instructables.com/FZJ/N71F/I7W1X2XO/FZJN71FI7W1X2XO.LARGE.jpg

Would a ratio of 10kΩ and 40kΩ be what I need based on 3v?