You could try this: get your average character width in pixels with fontmetric, then divide the width of the edit box by that. Set your memowidth to that number. Then you can use memlines.
so:

nAvgCharWidth=fontmetric(6, form1.edit1.fontname, form1.edit1.fontsize)
nMemoWidth = form1.edit1.width/nAvgCharWidth
set memowidth to int(nMemoWidth)

hth -jack