When painting, a painter uses his brush to judge distance to transfer the right scale of an object to his painting. When I started out at WTG, I used my thumb to calculate how hard I should hit, my length of the nail was 3.5ft. It was perfect for short distance putts.
Then I ate an ice cream, and started to use the left overs - the ice lolly - to control my putts. I painted few lines on it so I can easier see the scale. Instead of an ice lolly, paper or a yellow sticky note can be used just as good.
I thought that I share this idea with you all, as a extra tip to nivlacs already great finding regarding distance control when putting. Hope it helps.