How Do You ID Parking Garage Levels?

The only practical answer here is to ask the manager or employees how they refer to things, because ultimately they're going to be using the system. 

I remember when alphanumeric keypads started becoming more popular for residential burglar alarms. Instead of naming zones "north window" or "bedroom three", I would ask the homeowner what each room was called, allowing me to name zones "Stephanie's room" or whatever. I was surprised to see how much users responded to it. Increased customer satisfaction and cut down on false alarms (compared to numeric keypads that only showed the number of the zone). 

I struggle with this issue all the time and agree that asking the owner to provide location descriptions is the best solution. I find parking garage layouts so confusing that I often print out small scale versions of each floor plan on paper and then tape them together to create a 3-D model of the garage. This helps me to visualize where I'm at as I'm doing my design. I have gotten some flack from others as to why I am "playing with paper dolls" but I find this to be very helpful.

Prior to getting customer input, I would label everything up until you reach the next level as the next common number. In your example 1.5, 1.3, 1.75 are just 1, the decimal is meaningless in building terms of "Floors" or "Levels". If I needed to add additional references, I would use North/East/South/West as an accepted/measurable reference.