You don't have a fancy dive computer, you have a basic one. I don't know of any basic computers that have that functionality. Some more advanced computers have a planning mode that will show you NDL times at a user selectable Surface Interval (SI)....I was really hoping to find a "XX mins until tissues cleared" feature or something like that. Basically, I envision diving, having a surface interval, and being able to immediately know how long I have to wait before I can dive again without bumping into a modified dive plan because of residual nitrogen build up.
Yeah, yeah, dive tables and all that, but ain't this part of what my fancy computer is for?
BTW, it's neither helpful or even possible to accurately calculate the length of time it takes to fully offgas. From DAN's The Nitrogen Saturation Myth "Once a diver has returned to the surface, regardless of whether the dive was a short recreational dive or a long saturation dive, all tissue inert gas in excess of normal atmospheric pressure will be eliminated over the following 12 to 18 hours (in other words, his body’s tissue nitrogen levels will be re-equilibrated to the ambient atmospheric pressure)."
This isn't normally a problem for recreational dives. You just wait out the SI until your computer shows you the NDL you want at the depth you want. If you have a tight schedule to keep, then you should be using dive planning software to plot out the day's dives in advance.