Yup, your math is spot on if a mine is covering 3 clay and 1 stone block!

Basically, the formula for how to find the average output of a given mine is,

For each tile a mine is overtop of, add the length of that tile's process to the denominator of a fraction.

For each tile of the desired resource, add how many of that resource is produced per process to the numerator of a fraction.

For example, to calculate the average output of a mine covering 2 clay, 1 dirt, and 1 stone, it would look like this:

2 + 2 / 18 + 18 + 5 + 12 = 4/53

or .07547 clay/sec

Since clay is the desired resource, you add 2 to the numerator for each patch, and since dirt and stone are not, you do not add to the numerator for them.

Because this is the hypothetical average output of mines and not the actual output of mines, I usually have my mines produce about 5% more than required, to reduce the chance of being short resources. For example, a kiln requires 0.4 clay/sec, but you would usually want to make sure your mines have an average output of 0.42 clay/sec or higher to reduce the chance that your mines don't produce enough clay.