For those of you who need to adjust the width of a 16 column gantry layout I have attached a modified version of the mywidth.php as mentioned earlier in this thread.
In effect all I have done is to change all instances of 12 to 16 and where the margins total 240px in a 12 column grid, this needs to be 320px in a 16 column grid. This then calulates in the same way as the original 12 column version.
Just follow all of the instructions in Henning's post and substitute the attached file.
Remember - Backup and save files BEFORE you need them!