Its not really going to make that big of a difference. If you wanted to do the calculation, you'd have to figure out the points given by the starter and add that to the points given by the wort. For instance, if we assume that 2L is close enough to 2quarts, that gives you half a gallon of 1.040 wort which would give you 0.5 * 40 = 20points. In the full batch, you've got 6.5gal of 1.070 wort, or 6.5 * 70 = 455 points. If you added the two together, you'd now have a total of 475 points in 7 gal of beer, giving you a gravity of 1.068. I don't know about you, but I have a hard enough time with either my hydrometer or refractometer getting good measurements down to the .002, so my measurements probably contain enough uncertainty to make the adjustment unnecessary.
As for the rest, yeast shouldn't change the gravity since they're in suspension. The air might change it a little bit, but I doubt it would be enough to notice, unless its aerated enough to start getting bubbles on the hydrometer (like with CO2 after fermentation is done). I generally measure before I pitch the yeast, but that's just because its easier for me to grab some out of the BK than out of the carboy.