Fix slave allowed error calc

[DAVe3283]

The current method of calculating the allowed error on each slave board is based on how many chimes that particular slave is currently striking. This was done to work around an issue where the voltage would drop as the number of chimes struck at once went up, but it turns out I was just exceeding the rating of the power supply. Plus, they have no way of knowing how many chimes the other slaves (of which they also don't know) are striking, so this method is fundamentally flawed.

The master should control how many chimes can be struck simultaneously, and the slaves should just have a reasonable error limit (e.g. 2-5%). If a measurement is out of range, it is probably a good idea to shut down the power supply, because something is wrong. I just thought it wasn't.