Of course it's possible that tamox is more than 1 g/ml. We use 1 g/ml because it's simple (and the density of water), but if you have something accurate enough, then add half solvent, then powder, then rest of solven to desired volume.
Where do you guys get the 50 ml bottles accurate to 1 ml? Getpinz?
Here's what I do, (although I have some accurate glass pieces, this is easier so I don't have to wash them):
1). Draw the PEG out of it's container with a large syringe measuring 50ml.
2). Squeeze it into your oral solution bottle.
3). Now put your label on, making the top of the label your 50ml marker line.
4). Draw half of the PEG back into the syringe.
5). Add 1 gram of tamox powder.
6). Re-add PEG back up to the top of the label.
7). Put the remaining PEG back into it's original container.
8). Now relax and take a deep breath. It's time to clean up the random drips of PEG so your roommates/girlfriend/mommy don't start asking questions about mysterious substances on the counter/table/whatever.
If you think about it, even if you were off by as much as 2ml, which would be pretty sloppy, the end result would be less than 1mg/ml. And with some of the dosing schemes people come up with, that slight difference isn't going to make a metric ounce of a difference.