Oh... and (i think this is correct because I did it last year) - if you increase your estimate later on - you can choose to have your FTB and CCB 'adjusted' to avoid the debt as well.
So for example you estimate 70k combined income now and in four months you up it to 80k - not only will the FTB and CCB adjust to the new figure you are giving, but you can also elect to have more withheld to help it balance out - iykwim.
you have a "choice" to adjust CCB. it is mandatory for FTB to be adjusted - so always better to put in the higher estimate early