I use old wallets and gloves for leather, thinner leather with new flints, I also cut a diamond shaped hole in it . (large siler 3/4 x7/8 suggested by chambers & track of the wolf)
Brad at Cabin creek here in Pa put a 7/8x1 inch in my golden age (same as large siler) saying "you want to use all of the frizzen face for the best sparking and even face ware" he told me to nibble the rear of the flint if to long, ("you may have to notch the flints if to long, use plyers to nibble the back" )