the native script units are in points (72/inch) so google the ratio of whatever 1pt is in mm and multiply your .left and .top by them.
I just had to do this today, and it's 2.83465
that's weird I found this value pt > mm .35277777777777800000, I tried it and it is exact.
depends on which way you want to go
2.83465 pts = 1 mm
0.352777 mm = 1 pt