This is #2 for me as well, and my fundus is about midway to my bellybutton. Maybe a little higher than midway. It's my understanding that fundal height is always an estimate and not something to be heavily relied upon and especially unreliable before 24-26 weeks. Although, if you were having twins or more, then your fundal height now would be noticeably high...
I have a midwife textbook that has a nice diagram of where it should be every few weeks, but I found something similar on google. If I remember correctly, the fundus should be at the bellybutton around 22 weeks. And midway to bellybutton around 16 weeks. So, it seems like we both are measuring a bit ahead although I'll be interested to see how things even out further along in the pregnancy.
For measuring by yourself, I've heard (and use) the flat side of my hand (like where my pinky is) and beginning high up on my belly (while lying flat on my back) press down on my belly while also moving my hand in a downward motion. Your fundus will feel like a very hard ridge and will be fairly noticeable (if you know what to look for) once it's risen over the pelvis. You can then roughly measure it by counting finger widths up from the end of your pelvis up to the top of the fundus...that should roughly correlate with the number of weeks along you are...but again, that isn't very reliable until more like 24 weeks. You can also measure with a tape measure (of course).
Your midwife would have checked low down by the pelvis bone at 12 weeks because that's a common week where the fundal height is finally above the pelvis bone. She was probably just guestimating on where she'd have the best chance of hearing Baby. That said, I'd SWEAR my fundus was above the pelvis bone at 8 weeks!
I hope this helps!