It's not clear what you're trying to do that is causing difficulty. Typically, there is nothing to do... the fact that GDL interprets numeric lengths internally as meters isn't something that anyone usually needs to pay attention to.
For example, if your Working Units are meters and your parameter A is the chair dimension you want to be 450mm, then you would enter 0.45 for that parameter in the object settings dialog. If your Working Units are millimeters, you would enter 450. When the parameter is passed to the object script, the conversion is performed to meters. Same if an Imperial Units user entered 1'-6" in the object settings dialog (or in the script) ... that would be converted to meters if you were to examine the value received in the object script (0.4572 m).
If you are hard-coding your chair to be 450mm, then you would enter 0.45 in your script.
In case this isn't clear yet (I'm tired), let's say you have a parameter in your object that defaults to 450mm (so you would have entered it as 0.45 since values are meters inside your script). In the Object Setting dialog, if your working units are set to mm, that parameter will appear as 450. If you working units are meters, it will appear as 0.450.
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