instructions from fontlab
in the paragraph below to import a font correctly from illustrator
Still in Illustrator, select File > New. Set the width of the document in
points to be the double of the UPM size of your font (e.g. 2000 pt for a 1000
UPM font). Set the height of the document to be the same as UPM size –
Descender (e.g. 1000 – (–263) = 1263 pt). Select Window > Info, View >
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Typically TrueType fonts have UPM (Units Per eM – the size
of the grid on which all glyph coordinates are defined) equal to
2048. Type 1 fonts have UPM equal to 1000.
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to clarify here is were im confused
Set the width of the document in
points to be the double of the UPM size (mmm ok wot is the upm size is that double the size of the grid the font is made on)