Hearthstone's ranking system is inflationary: the total number of stars that players have increases over the course of the season. This leads to a situation where players of a given skill level should increase their rank over the course of each ladder season without actually getting any better. This is also why the ladder has to be reset so often; if it just ran unattended for too long, nearly everyone would be legendary.
So, unfortunately, you cannot measure skill by ladder level. You would have to measure it by ladder level a given amount of time past the start of a season.