Years ago on this very board I got a formula for scale effect which told: "add lightening color -white- in a percentage equal to the square root of the denominator of the scale".
Given for 1:350, the denominator is 350 and its square root is 18.7 so according to this formula I should add about 19-20% of white.
I absolutely DON'T KNOW the origin of this formula, but it seems to work fine, except maybe for Navy Blue, for which color I tend to add up to 30% of white.
For scale effects on lighter colors like white, yellow and red I add about 3 to 5% of neutral gray and I can live with this.
Maybe there are other formula which work as well. But since I use this my builds no more look like toys optically.