Measure string till caret position

Hello guys,
In my current project, I'm making a control similar to RichTextBox. I use TextRenderer to draw text. My problem is that I can't get real column index from caret position.

Here is my code:

Code:

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//row index  -works great
var y = e.Y / LineHeight;
if (y >= lines.Count) y = lines.Count - 1;
if (y < 0) y = 0;
columnCount = y;


//column index
int x = 0;
int i = 0;

for (i = 0; (i < lines[columnCount].Length) && (x < e.X); i++)
  x += TextRenderer.MeasureText(lines[columnCount].Substring(0, i), Font).Width;

if (i > lines[columnCount].Length) i = lines[columnCount].Length;

rowIndex = i;

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It returns something like half of the string :) Anyone has a solution?

iRoN