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 Ivory Black first: it is 69% of the mix.
Add the Cadmium Red Scarlet 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.
Judge the color in the light you will paint in, and test a dab on the canvas: wet paint and dry paint can read differently, especially acrylic, which usually dries a little darker.
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.