What is the best oven mortar mixture for the bricks in the dome?

[shingletown paul]

I am not a mason a DYI at best but I do enjoy my projects Most of my research has come from a book “The Bread Builders” by Daniel Wing & Alan Scott. Thus far I have completed the foundation, block wall, Hearth slab insulation and the heath slab. This week I plan on laying the hearth firebricks and the oven walls. The book indicates using refractory mortar or mortar mixture of: 10 parts mortar sand, 3 parts Portland cement 1 ½ part fireclay for the oven walls. According to the book this mixture should provide hardened mortar with an expansion rate similar to the firebrick as well as a sticky usable mortar. Looking at the forum it appears there are a lot of different opinions to the mortar theory I do not want to get all the cladding, insulation and façade complete only to see my oven mortar fail.