ALL of SketchUp's internal database lengths are stored in inches, irrespective of the units you use.
In the API there are the numeric and length classes.
So if you have an input value that defaults to a 'length' - e.g. 9.feet, or any other units format - then it is always 'stored' and 'processed' in inches [9*12=108"].
However, this is flexible as the user can input in any units provided they use a units suffix: however how it's displayed in the input-box depends on the current model info > units settings...
Otherwise you need to default to a string value and then reverse-engineer that - e.g.
h="9" [read from the input[1]]
which you then reset for later use as a parallel value
hft=h.to_f.feet
So the input is always taken as a decimal-feet value, but then used in inches internally...