Constantly adding 50/50 is something you should not do ... the PG half of the mix does not evaporate readily ... what it does is regulate the rate at which the distilled water half evaporates ... if half of the water evaporates and you replace it with 50/50, your mix is now 62.5/37.5 ... as you might imagine, if you continue that pattern, you will end up mostly PG and very little water ... and that will not properly humidify your cigars ...
Instead, charge the credo with 50/50 and add plain old distilled water as needed, using the 50/50 very sparingly.