How do I calculate the humidity inside sealed containers containing mixing salts? At $\pu{25 ^\circ C}$:

saturated Magnesium chloride $\ce{MgCl2}$ keeps humidity 33% inside sealed containers.
saturated Sodium chloride $\ce{NaCl}$ keeps humidity 75% inside sealed containers.

I want to keep humidity at 60%. Which ratio should I mix this two chemicals?
How we can calculate humidity for mixing salts? Is there a formula or table list for this?
At $\pu{25 ^\circ C}$ temperature, $\ce{NaCl}$ sets 75% humidity and $\ce{KCl}$ sets 85% humidity. NaCl mixed 1:1 with $\ce{KCl}$ will give an RH% around 70–71 between $20$ and $\pu{30 ^\circ C}$.
What is the formula to calculate 70% with using 75% and 85%?
As Ivan Neretin pointed out in his comment, there is no simple formula on this matter. Howevr, if you have access to ammonium nitrate ($\ce{NH4NO3}$) or sodium nitrite ($\ce{NaNO2}$), saturated solution of both give 64% relative humidity (RH) at $\pu{25 ^\circ C}$ (Engineering Toolbox).
Interestingly, magnesium nitrate ($\ce{Mg(NO3)2}$) gives exactly 60% RH at $\pu{0 ^\circ C}$, but it was only 54% at $\pu{25 ^\circ C}$. A few salts and their measured RH values in different temperatures are given in the following Table for readers' convenience:
$$
\bf{\text{Relative Humidity over the Saturation Salt Solution (%)}}\\
\begin{array}{c|ccc}
\hline
\text{Salt} &amp; \pu{0 ^\circ C} &amp; \pu{10 ^\circ C} &amp; \pu{20 ^\circ C} &amp; \pu{25 ^\circ C} &amp; \pu{30 ^\circ C} &amp; \pu{40 ^\circ C} &amp; \pu{50 ^\circ C} &amp; \pu{60 ^\circ C}\\
\hline
\ce{NH4NO3} &amp; - &amp; 75 &amp; 67 &amp; 60 &amp; - &amp; - &amp; - &amp; - \\
\ce{(NH4)2SO4} &amp; 82 &amp; 82 &amp; 81 &amp; 81 &amp; 81 &amp; 80 &amp; 79 &amp; - \\
\ce{MgCl2} &amp; 34 &amp; 33 &amp; 33 &amp; 33 &amp; 32 &amp; 32 &amp; 31 &amp; 29 \\
\ce{Mg(NO3)2} &amp;99 &amp; 98 &amp; 98 &amp; 97 &amp; 97 &amp; 96 &amp; 96 &amp; - \\
\ce{KNO3} &amp; 96 &amp; 96 &amp; 95 &amp; 94 &amp; 92 &amp; 89 &amp; 85 &amp; - \\
\ce{KCl} &amp; 89 &amp; 87 &amp; 85 &amp; 84 &amp; 84 &amp; 82 &amp; 81 &amp; 80 \\
\ce{NaCl} &amp; 76 &amp; 76 &amp; 75 &amp; 75 &amp; 75 &amp; 75 &amp; 74 &amp; 75 \\
\ce{NaNO2} &amp; - &amp; - &amp; 65 &amp; 64 &amp; 63 &amp; 61 &amp; - &amp; - \\
\hline
\end{array}
$$