Suggestion for use of Penetrol with Spar Varnish and Miniwax Cherry Stain?

I am Carving a Maple Tree, creating an Abstract Sculpture. I heard that Penetrol enhances the use of Spar Varnish. I was also wondering if it works with Minwax Cherry Stain (I want to get a darker Cherry Stain to the Maple Wood.

What ratio of Penetrol to Varnish should I use.