The creation of an under-hearth foundation that does not sit flush with the outside walls is a distinct step in this plan that is unusual from general oven foundation plans. This may be done to ease the process of creating an in hearth opening for wood ash or minimise the firebricks use. Generally it is recommended that the hearth foundation be laid to sit fully against the outside walls thus reducing the eventual finishing work required. To produce this larger foundation simply create a larger temporary wood form which sits outside and flush against the besser block walls. Support and level this form with 2" by 4" legs and shims as with the internal sheeting structure. This would result in a 1800 x 1800 x 200mm foundation rather than 1400 x 1400 x 200mm.
Partial foundation frame
Complete foundation frame
Confusing. Dimensions say the slab is 1300mm X 1200mm but Step 1 says the slab is 1800mm X 1800mm