Symbol position

[Natter]

There is a TSAY control :

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@ 20,20 SAY oCl  PROMPT "Some text"  PIXEL  OF oDLG  SIZE 100, 20  FONT oFont

The text is pressed to the left. In this case, I can easily determine the starting position of any character.
For example, the 6th -"t"

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GetTextWidth(NIL, left(oCl:GetText(),5), oFont:hFont)+1

However, if the text in TSAY is centered,

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@ 20,20 SAY oCl  PROMPT "Some text"  PIXEL  OF oDLG  SIZE 100, 20  FONT oFont  CENTER

then the position of the symbol cannot be calculated in this way.
How can this be done ?