Module name - confused

[Kyoku]

Hello,

I've created a package named : SLS
I've created a module in this package named : a_Calls
I've added in this module a field named : Date_Call, type date, and i want to change it to datetime.
I read I have to create a custom.php file like described here : http://www.sugarcrm.com/forums/showt...ardefs.ext.php

I'm little confused with the module name and the exact directory name i have to create this file :
In studio, i see my module named : SLS_Calls (which is the Label name I've set in module builder)
In module builder, i see my module named : a_Calls
In repair & rebuild, i see my module named : SLSPK_a_Calls (SLSPK is the Key i've setup in module builder).
So, where I have to put the file custom.php :
1- custom\Extension\modules\a_Calls\Ext\Vardefs\
2- custom\Extension\modules\SLS_Calls\Ext\Vardefs\
3- custom\Extension\modules\SLSPK_a_Calls\Ext\Vardefs \
??

Thanks for any help.