ndl calculation

I want to make a diving decompression algorithm. I read a couples of papers but 'winke's yet. so I understand about what is m-value, how to apply gradient factor, how to calculate a tissue compression, what is NDL and how can calculate it, what is water vapor, and difference of compression about...

Had another great dive today, but was a bit puzzled by my computer. At ~25m or so, I ran out of NDL, and as I've usually been doing, I went up a few meters. Usually that gives me quite a bit of additional NDL-time, but not so today. Although my "ceiling" never went below 0,2meters, it puzzles...