Annotative Mtext

I have almost got the hang of annotative scaling which seems very useful, but stuck with one part and someone might be able to advise.
The width of the edit box for mtext does not indicate the true length of a word wrapped line as it does for non-annotative mtext.
I can stretch it to a fraction of the true text wrap width with grips to wrap correctly, but some strings don't keep the same width on reopening the file.
Some mtexts can have a very wide edit box, but still be word wrapped.
There is an editable 'paper defined width' property, but the most successful approach seems to be make the box wider than the text and manually add line breaks.
Paper Defined Width can also be 0 for a block of mtext wrapped correctly, making it more confusing.
Any help would be appreciated.