You can write a subroutine to determine the RPM from the z value. If to the nearest 100rpm is acceptable, this is the relevant data:

Code:
z           rpm
-------------------------
<10         out of limits
10          6000
11          5500
12          5000
13          4600
14          4300
15          4000
16          3800
17          3500
18          3300
19          3200
20          3000
21          2900
22          2700
23          2600
24          2500
25          2400
26          2300
27          2200
28-29       2100
30          2000
31-32       1900
33-34       1800
35-36	    1700
37-38	    1600
39-41	    1500
42-44	    1400
45-48	    1300
49-52	    1200
53-57	    1100
58-63	    1000
64-70	    900
71-80       800
81-92       700
93-109      600
110-133     500
134-171     400
172-240     300
241-400     200
401-1200    100
>12000      out of limits
If you need help on how to write a subroutine to do this then let me know and i can help you out.