Compare your rates with market averages.
A rate gap of a few units per hour looks trivial until it is multiplied by a year of billable hours. Adjusting the market median by the premium your experience justifies gives a defensible target rate, and the annual gap is the argument for moving toward it. Because the multiplier is a thousand hours or more, even a modest underpricing compounds into a serious sum.
Rate gap
Target rate = market median x (1 + premium/100); yearly gap = (target rate - your rate) x billable hours per year
Industry rate surveys, professional association reports and job-board ranges for comparable contract work. Treat single anecdotes as noise.
Usually in steps: new clients at the new rate, existing ones at renewal. The annual gap shows what the delay costs, which helps set the pace.
Then the gap is negative and you are pricing above the median, which is fine when positioning, demand and results support it.