Hello Gerald,
unfortunately you only can set an integer value via UMC. If you have a testsystem you can try the following commands on this system as a workaround. This will overwrite the value you have to set in the UMC anyway.
pkprinters -c 0.0082 <your_printer_name>
You can check the value with
pkprinters -l
and
repykota
You are aware of the fact that you have to enable print quotas and this only works for the lpd, ipp, http, socket and smb protocols.