I haven't looked over your math in too much detail, and other commenters have you covered on the limitations of floating points (and have suitably chastised you for not using the metric system), so I'll leave all those point aside and just throw out one piece of information that I learned that hard way about Modicon PLC's and Proworx 32, in case it's contributing to your problem.
The logic is solved vertically, not horizontally.
Yes, really.
Code:
| +-------+ +-------+ +-------+ +-------+
|------| ADD |------| ADD |------| ADD |------| ADD |
| | | | | | | | |
| | | | | | | | |
| +-------+ +-------+ +-------+ +-------+
|
| +-------+ +-------+ +-------+ +-------+
|------| SUB |------| SUB |------| SUB |------| SUB |
| | | | | | | | |
| | | | | | | | |
| +-------+ +-------+ +-------+ +-------+
|
| +-------+ +-------+ +-------+ +-------+
|------| MUL |------| MUL |------| MUL |------| MUL |
| | | | | | | | |
| | | | | | | | |
| +-------+ +-------+ +-------+ +-------+
If the above code were any other PLC, it would execute all four ADD instructions, then all four SUB instructions, then all four MUL instructions.
In a Modicon PLC, it'll execute the first ADD, then the first SUB, then the first MUL, then the second ADD, SUB, and MUL, and so on. This can completely mess up your calculations if you put your blocks in the wrong place on the screen. It gets even more complicated if you have regular conditional contacts in series, because they're still connected left-to-right as you'd expect. So once they're executed and determined to have a true or false output, the PLC trots off to the next line to see what else is happening, before returning to determine what to do with the aforementioned true-or-false condition that it worked out a couple of ticks ago.
Once I found out about this "feature", I started doing all my math like this:
Code:
| +-------+
|------| ADD |
| | |
| | |
| +-------+
|
| +-------+
|---------------------| SUB |
| | |
| | |
| +-------+
|
| +-------+
|------------------------------------| MUL |
| | |
| | |
| +-------+
|
