The recipes below come from Paintwell's pigment model, worked out from the real tube paints named, in oil and in acrylic. Percentages are by volume of paint on the palette.
Put out the Radiant White first: it is 75% of the mix.
Add the Scheveningen Blue Deep and Cobalt Blue a little at a time, mixing fully between additions. The smaller shares are usually the stronger paints, and it is far easier to add more than to take it back.
Because sky blue is a light color, work the darker paints into the lighter one, never the other way around, or you will end up mixing a pile far bigger than you need.
Every brand's version of a pigment is a little different, which is why the recipes change from brand to brand. If you own other tubes, Paintwell works the recipe out from those instead.
Take a photo of your reference, tap any color, and Paintwell gives you the recipe from the paints you actually own. Then point your camera at your mix and it tells you which way you're off.