Sketchy recursion in Inception

In the movie Inception, most of which takes place inside people's dreaming minds, we are told that 5 minutes of dreaming equates to 1 hour inside that dream (a 1:12 ratio) because "your mind functions more quickly."  If, once you are dreaming, you go to sleep and have a dream (within a dream), the same ratio holds, so that 5 minutes in the "real world" equates to 12 hours in the 2nd-layer dream.

We are told that the more layers of dreams-within-dreams you have, the more unstable your situation becomes, so the pharmacist administers some powerful sedatives to help.  But this changes the ratio from 1:12 to 1:20 somehow, and now 5 real minutes should give you 100 minutes in the first level of dreaming, 2000 minutes (33) hours in the 2nd level, and so on.

The numbers given in the movie were a bit sketchy, though - instead of "100 minutes, 33 hours, 28 days," they said something like "2 hours, 1 week, 6 months," or something like that.

The other fishy math was the amount of dreaming that got done during the time the van was falling from the bridge to the water.  If we're generous and say the bridge was 100 ft above the water, it should have been about a 4.5-second fall.  The should have given them 90 seconds in the hotel and 30 minutes in the snowy wonderland.  Max.

Do I have this right?