Yes, the first bubbles will be mostly air. In a 5 gallons fermenter, many hundreds of thousands bubbles will go through the airlock and the ‘headspace-bubbles’ will have little significance to the overall calculations.
This small error will vary depending on how much headspace you have in your container, and we decided not to add this as a user input, as it will complicate the product more than you'll gain in accuracy.
"There is an initial lag phase lasting some 12-24H in which there is little observable increase in yeast biomass or decrease in wort specific gravity. In this period oxygen is rapidly taken up by the yeast and anaerobic conditions pertain for the remainder of the fermentation."