Confused about when I'll miss my period?
You are correct. If you had a period in January you are not pregnant from sex at Christmas.
If you got your January period, you are not pregnant. But since you are clearly wanting to BE pregnant, you presumably have had sex i January as well, yes? So if you missed your February period, you could be pregnant.
If you got pregnant on December 26-27 you should have missed your period around January 5-10 based on your ovulation date. If you had that period you should not be pregnant and are probably just late on this period. You say your period varies as much as 5 days normally, so ...
And how have you pinpointed ovulation
There is no one size fits all. You may have a period the entire pregnancy or have your next period but skip the others. Normally you would have your next period but it would be very light and spotty.