Due Date

I’m a little confused on how to calculate your due date. My first ultrasound isn’t until March 21st and I know you can calculate it based on the first day of your last period. However, on this app it asks for the first day of last period and how long your prior cycle was and that’s where I’m getting confused. My last period started on Jan 25th and my last cycle was 24 days long. Based solely on my lmp it says I’m 4 weeks 2 days (Due Nov 1), but with the last cycle length it says 4 weeks 6 days (Due Oct 28). First timer here, sorry!