As the adventurers get deeper into the dungeon, you roll dN and you add the appropriate modifier for the distance from the entrance.
I put in a formula (untested) that randomizes the modifier from -10+(half the depth) to (half the depth). So, if you were 6 rooms in, it would randomize a modifier from -7 to 3. I just thought this might be easier than making up a line graph for each dungeon.
I could probably put in cells to tweak the formula.... That'll be V.2. :)
Anyway, you make up the four charts and you roll a d10 and add the modifier. You can hit CTRL-R to recalculate the variables.
Here's a screenshot.
Here's a link to the GoogleDoc. Save a copy and try it out. Thanks again Zak!