When I tinted my SOF I used less than a tooth paste sized tube of artist oils ... on the order of 1/2 as I remember.

Note though ... that sometimes getting the color you want by tinting varnish with artists oils isn't so easy as it seems. It depends on the color your shooting for and the type and quality of the pigments of the artists oils. Once the oils are mixed with the varnish the color of the pigments becomes more apparent. You can start with a dark brown artist oil and end up with an orange color when it's mixed with varnish. It can be a real hit and miss affair depending on the color you're after. Mixing different colors can be even harder.

I'd recommend making some test swatches before you try to tint a full can of varnish. Also remember that the more layers ya have the darker the color will be.

KenB