take the measurement of your overall outside width of the tank. Take the measurement of the pane of the glass that goes lengthwise. now subtract that number twice from your overall width and that is your inside dimention.
my 29gal tank is 12-1/4" over all width. the glass panes are 1/4". i just got my glass baffles cut. the 11-3/4" are perfect, the 12" ones are too big by (you guessed it, 1/4")
you can't guess at this, you actually have to measure for yourself and do the math.