I'm having a similar topic also.
They do estimate dd from the day of your last period.
I had hormone therapy in mid may.. (had a Iui done).. I was on hormones, grew follicles within 8 days and inseminated by calander day 10, then got a pos hpt another 10 days later....
"technically" I'm 4 weeks since my lmp, though I disagree...
So my cycle this month was 24 days-not 28
Hope this makes sense