As you know load capacitance and impedance will affect the shape of you clock and the sources of most new devices are lower than most cables. i.e.74ALC ~ 25 ohms so in order to manipulate the same shape, you need the same impedance ratios. Since you have no specs, all I can suggest at the moment is to use different impedances to drive the fan out inside and the external load so they have the same prop. delay if they have the same threshold. e.g. use Rs internal series R ~ >=25ohms and external use Rs = 50 ohms to match cable then use pull-up/down or active termination R to optimize the external clock to the threshold.