regression for the colour-age relation (thick grey lines in Fig. \[figure:ubt\]): $(U-B)_0^i\sim0.5\log t$. Evolutionary tracks with sub-solar and solar metallicities in both models satisfy this regression in the range of ages from $\approx15$ to $\approx160$ Myr. Based on more modern CMD model, we calculated ages of star clusters using the relation $\log t {\rm [yr]} = 2(U-B)_0^i+8.6$. The linear regression between age and colour allows us to minimize errors associated with photometric calibration for the $\log t - \log S$ slope estimation.

We used several key separation values. Two of them are characteristic scales of hierarchical star formation (100 pc is a typical size of OB-associations and 500 pc is a diameter of star complexes). The other two boundary values of $S$ were selected from the observations (40 pc is a minimal linear scale of our observational resolution and 200 pc is a maximal distance where we observe the correlation between the colour and the separation visually in Fig. \[figure:col\_dist\]).

As a result, we chose three separation ranges for the further study of the $\log S - \log t$ dependence: an inner scale from 40 to 200 pc, an outer scale from 100 to 500 pc, and a general scale 