I was digging through the Help files today looking for blog post inspiration (after nearly 300 tips & tricks, it can be hard to find...who wants to send me a tip?) and stumbled across SPACETRANS.
In LT 2010 at least, SPACETRANS is on the Text toolbar, but of course you can always type it in or make your own button.
According to Help, SPACETRANS
Apparently this command is for when you're working either in a layout or in a floating viewport (i.e. you double-clicked inside the boundary of a viewport). Don't try to access it in modelspace, 'cause it won't work there.
If you're in a floating viewport and run SPACETRANS, you're prompted to enter a paperspace distance, which is then converted (based on the viewport scale) into the equivalent distance in modelspace.
If you're in a layout, you have to pick a viewport first, then enter a modelspace distance to be converted into a paperspace size.
SPACETRANS can be (and usually is) used transparently, as 'SPACETRANS. (Notice the apostrophe before the command name.) Using a command transparently means you enter it instead of a number when prompted for a height or length.
For example, here's how you might use SPACETRANS to calculate a text height:
Specify start point of text or [Justify/Style]: (pick point)
Specify height <0.500>: 'spacetrans
Specify paper space distance <1.000>: 1/8
Resuming TEXT command
Specify height <0.500>: 1.5 (if the viewport scale were 1:12, say)
Just another way to let AutoCAD LT do viewport-scale math for you.