my lmp date puts me at 8/28 (I had short cycles - 25 days).
Without knowing ovulation date, your normal cycle length doesn't help much more than LMP does.
Back-calculating, I come up with LMP of 11/21, and "assumed" ovulation / conception (as opposed to the date the sperm came into play, which could be as much as 5 days before ovulation, another reason that saying "I know my dates" when you don't know when you ovulated doesn't really work) of 12/5.
But assuming that this would have been a typical 25-day cycle (maybe a reasonable assumption, maybe not), and that you have a normal luteal phase of somewhere between 10 and 16 days (a reasonable assumption, but not very precise), conception would be somewhere between 11/30 and 12/6, with 12/2 a most-likely conception date.
Calculating forward again, that gives you a possible 40-week date somewhere between 8/23 and 8/29, with 8/25 being most-likely.
two ultrasounds put my due date at 8/18, which is what we've been going with for my own sanity.
Early ultrasound can date a pregnancy pretty accurately - within a couple days for a 6 or 8-week ultrasound.
If this was a 20-week or later ultrasound, it can't be used for dating - the "date" it gives you is "measurement equivalent to 50th percentile of babies of that gestational age." It would be like saying your baby was 10 pounds at its 6-week well check, which was 50th percentile for a 4-week baby, so instead of writing "10 pounds" in the medical record, we're going to write "4 weeks old."
If your 40-week date per ultrasound was 8/18, that means you would have conceived on 11/25 - so on CD 4. Which is extremely, extremely unlikely.
I'm not sure what you mean by the "for my own sanity" bit. The baby comes when it comes, assuming no medical reason to make it come earlier, and expecting the baby to come earlier because you choose to think of it as due earlier is just going to make the last weeks even worse.
Doctor says this baby will be huge
When I was doing my research when pregnant with DD6 (so 6.5 - 7 years ago, which is an eternity ago for medical stuff) "doctor thinks baby will be big" was a much better predictor for complications and interventions than "baby is actually big" was. Odds are, a second baby will be larger than the first baby was. But positioning (both mom and baby), speed of delivery (time for everything to stretch out), and weight distribution (fat squishes, skull, not so much) are bigger factors for ease of delivery than weight per se.
Do you think this baby is going to be big? The last I looked (which, again, was a long time ago), second-time moms were better predictors of their babies' weights than are either ultrasound measurements or health care providers.
If my calendar date would be the 28th, aren't I already shorting him 10 days by looking at the 18th?
If your doctor has "due on the 18th" on your chart, and thinks this is going to be a big baby, I think you're going to get a lot of pressure for induction well before the 28th, assuming the baby doesn't come on its own before then. I wouldn't anticipate any baby-complications (in that a 39-week baby is likely to have no issues), and with a second baby, it's less likely that you'd fail to respond to the induction and end up with a C-section. I personally wouldn't be happy with a 39-week induction, but you've got a high likelihood of a good outcome if that's what you end up with.
Thanks for all the help and keep them comming, its fun and I enjoy reading the behind the scenes stories of everyones luck! 
