"Is there a way that Moodle will calculate the correct number of significant figures automatically?"
Not in the actual (and future) moodle calculated question code.
Significant figures is a conventional way to express "approximately" the precision of real number calculation results.
There is no "official" rules but more or less common pratice i.e. using or not underscore to show the last figure as 7.05
The calculated question offers to the teacher the possibility to control the number of significant figures that will be used to display the good response.
It is left to the teacher to control the display of the numbers used as parameters to calculate the response so that they agree with the significant figures used to display the good response.
This is why when editing the calculated questions the good response is displayed.
Moodle also verify that the good response is within the tolerance limits set for the answer.
The tolerance limits are NOT calculated using the GOOD response but within the limits of PHP precision ( i.e. 14 digits) using the EXACT response of the formula.
ex formula = 10/6
The EXACT response is 1,66666666...
You have to be carefull on how you write your question text and how your set your tolerance and your good response.
Moodle can just handle the student response within your parameters.
Pierre
P.S. The tolerance is set using the EXACT result which is necessary for calculated questions that do to involved precision. The PHP limit in handling numbers which is related to the number of digits used to handle numbers in your specific installation is added to the tolerance that you set, so that PHP will not grade a good student response as a bad one.
This explain the length on the MIN and MAX values.