Promash needs to know the Brew-house efficiency to convert the amount of grain used into the amount of extract that will end up in the wort. Since this efficiency needs to be determined by brewing a batch, a default is given to help you designing your 1st batch. Once you brewed your 1st batch, you will get a more accurate value for your system (78% in your case).
bear in mind, that the efficiency depends not only on your system, but also the type of grain (e.g barley vs. wheat malt), sparging technique, OG (higher gravity beers may have a lower efficiency since you may have to stop lautering before almost all of the sugars are rinsed out), mashing schedule ....
But these differences will only be a few percent points in your efficiency. Once you have done more batches you will get a feel for the efficiency on your system and you can better predict the efficiency for future batches. But for now, since both are close, just work with the default 75% or your last efficiency of 78%.