How to anchor a text box to a table cell in word interop

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interop

MS-Word

Using C# interop for Word, I can create a table with 5 columns and several rows. I want to insert 2 things into column 5 of a particular row: a number representing a percentage, and a text box. The text box's width is equivalent to the percentage. The text box will be colored and will not contain any text.

My problem is that when I create the text box, it is always anchored to the first column of the row, not the column I request. The offsets (.1") also are calculated relative to the first column. I need to have the text box anchored to the last column.

If I manually edit the document, I can move the text box's anchor to the right cell, and everything looks OK.

Here's the code. It is invoked with [cell] equal to the last cell in the row. That value is correct because the text of the percentage shows up in that cell.

private static Word.Cell SetCellPercent(Word.Cell cell, decimal goal, decimal needed, bool? bold)
{
decimal pct;
float pctfloat, wid, hei;
Word.Shape box;
Word.Range rng;

pct = CalculatePercentDone(goal, needed);
SetCell(cell, (pct * 100).ToString("N0"), null, bold); // Store text into cell and apply attributes

pctfloat = Convert.ToSingle(pct);
wid = BOX_WIDTH * PPI * pctfloat; PPI = Points Per Inch = 72
hei = BOX_HEIGHT * PPI;
rng = cell.Range;

box = cell.Range.Document.Shapes.AddTextbox(Microsoft.Office.Core.MsoTextOrientation.msoTextOrientationHorizontal, .1f * PPI, 0.1f * PPI, wid, hei, rng);
return cell;
}

What I have tried:

I tried other values for [rng], like setting it to the range of the first and last paragraphs in the cell. Same result.