cenetti:
First, welcome to the wonderful world of film development.
You indicated at first that your "negatives turned low contrasty and I think underexposed".
Essentially, the development has a large affect on contrast, and only a small amount of affect on exposure. So if the exposure is too low, it isn't likely to result from a problem with the development.
You also indicated that you checked exposure with both a sekonic meter and your D90. It may be that your two meters were correct, but your camera is out of adjustment (shutter speed faster than set, or aperture not accurate).
You also state that your negatives came out "dark and low contrasty". Do you mean that your negatives were dark, or did you mean to say that your negatives were actually light (thin) which led to a dark scan?
Finally, scanners aren't a good way to learn to evaluate a negative if you don't already know how to evaluate a negative. I know that sounds strange, but unless your scanner and software are calibrated to give good scans when used with good negatives, then a "no adjustment" scan may actually lead to a poor image on the screen when the negative is good.
In a perfect world, you could show your negatives to someone who has experience with negatives, and they could advise you if you should change your exposure and/or development. Unfortunately, the Internet isn't a perfect world
.
In the meantime, here are a couple of questions:
a) are there a range of tones in your negatives, from almost clear (thin) to almost opaque (thick)?;
b) with respect to the thick parts of the negative, if they are supposed to contain observable detail, they should be slightly transparent - enough to be able to read black and white text from a newspaper through them. Are your (thick parts of the) negative like that? If not, how do they differ?;
c) how does the text on the edge of the film (the rebate) appear? Can the letters be easily read?
As for the general appearance of the negative, you might want to look at this site:
http://www.ephotozine.com/article/Assessing-negatives-4682
Hope this helps.
Matt