shorten file names

I want to shorten my mp3 filenames to 46 chracters. It's for my mobile phone. There's an option to shorten filenames to 64 characters in convert/actions but i can't edit that option. So if someone can help me i would be very grateful.
Allready figured it out with help from an answer from detlevd

Could you let me know how you did this ???

I need to convert file name to 16 characters for my cars head unit.

I use convert filename filename,

In the first box I put : %1 - %2
in the second box i put: %1 - $left(%2,$ifgreater($len(%1 - %2),45,$sub($len(%2),$sub($len(%1 - %2),45)),$len(%2)))

So you have to chance 45 with 16.
Mind you my file name are like this Diana Ross - Chain Reaction.mp3 so %1 is for the first part en %2 is for the second part. If your filenames contain more parts you have to work with %1 - %2 - %3 -%4 and %1 - $left(%2,$ifgreater($len(%1 - %2 - %3 - %4),60,$sub($len(%2),$sub($len(%1 - %2 - %3 - %4),60)),$len(%2))) - %3 - %4
Here ofcourse yoy have to change the 60 to 16 en even maiby to 12 because of .mp3
Anyway, you have to play around with it.