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If the field's fixedLineHeight is false you may want to use the formattedHeight of the field, which will account for any variances in the textHeight of lines within the field.
That assumes the goal is to obtain a number to be used for other purposes; if the goal is just to make sure the field is scrolled to show the bottom-most line of text you can use some unusually large number for the scroll and it'll stop at the bottom of the available text:
set the scroll of fld "MyField" to 999999
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function MaxScrollbyFH pFld
put the num of chars of fld pFld into nb
put the formattedHeight of char 1 to nb of fld pFld into pF
return pf
end MaxScrollbyFH