[support] Ubercart with custom price and attribute price multiplier

Hello, I need to configure an attribute (a select width 0.25 0.5 0.75 1.0 1.25 ... ) as a multiplier of the product price. I'd found info about this but can't get this to work (I'd checked permissions) http://www.ubercart.org/contrib/10772#comment-42778 I'd installed uc_custom_price and can't see where to place the custom price calculator. Any idea? Thank's!