How to pre-select default units

I use this module to allow the end user to select the unit of their choice inch or meter ...
But I have difficulties to make one units the default units intead of having the -- in the select list.
Is it necessary to make a hook_form_alter to set this and how.
Or is their an interface option that I miss.
I've tried to set the default units in default value of the cck field form, but it doesn't work all the time.
Thanks for your help.