How to determine if two calibration curves are parallel, within 5%
Define the angular coefficients of the two resulting lines and evaluate the parallelism between them. Perform the statistical evaluation of the data.
For the parallelism test between the curves, considering the equations of the lines to be compared as: $y_1 = a_1 + b_1x + \varepsilon$ and $y_2 = a_2 + b_2x + \varepsilon$, the hypothesis for comparison of the lines slopes is given by:
$$H_0 : b_1 = b_2$$
$$H_1 : b_1 \neq b_2$$
When the null hypothesis ($H_0$) is true, the regression lines become: $$y_1 = a_1 + b_3x + \varepsilon$$
$$y_2 = a_2 + b_3x + \varepsilon$$
Considering a significance level of $5\ \%$, the null hypothesis will be true when the test returns a P-Value above $0.05$.

More information in the link:
http://www.real-statistics.com/regression/hypothesis-testing-significance-regression-line-slope/comparing-slopes-two-independent-samples/