MP3 VB length tag issue

I'm having a problem with the length tag for my variable bitrate MP3 files.
Basically for some of my files the length tag is missing (either no value or 0). In trying to fix this I noticed that if MP3tag is set up to display the length tag as a time value (I think it's the default view for the length is this where the value/field is %_length% instead of %length%) the correct time length of the tracks are shown.
Basically I'm wondering if I can use that value to get/store the correct length number for the tag. Does anyone have any idea about this or what I'm talking about or am I crazy?

You can use a Format value action using LENGTH as field and %_length_seconds% as format string.