I don't guess there really is a 'proper' length for running one like that since it's a custom hookup.
Like mine for instance, I'm running it down the front of the driver's side crossmember/lower control arm area and back to the starter/solenoid next to the driver's side engine mount bracket. I'll cut the length as needed to get to the battery terminal.
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Greg Reid
Palmetto, Georgia
 
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