Any of you mommas out there on your second baby unsure of your due date?

When I was done pumping for my big guy I got two cycles back. It was 35 days in between the two. When I calculate my due date it’s asking for average cycle length. I have no idea what to pick because I don’t really have an average. Do I chose 35 or 28? Honestly 28 looks right when I look at my fertility friend and possible fertile days but I’m stumped. First appointment isn’t until Jan 16th! Thoughts anyone?