The data is subtracted by the mean of the wild type.
The sum of these zero-centered replicates plus the mean of the wild type is the theoretical mean assuming that these values are additive (i.e. have no epistasis).
The theoretical standard error is calculated by taking the square root of the sum the variances of the sets of replicates divided by number of replicates.
If the epistatic effect is beneficial, it is positive (+), if detrimental negative (-). Depending on the contributing values, there are different types of epistasis.
When the beneficial or detrimental gains of the base mutations simply stack by summation, (ie. without epistasis), this is additive.
When base mutations with the beneficial and detrimental gains are combined, but the result is greater than simply additive, this us sign epistasis.
When the beneficial or detrimental gains of the base mutations stack more than expected, this is magnitude epistasis.
When the beneficial or detrimental gains of the base mutations result in the reverse effect, this is reciprocal sign epistasis.