If they can get a 5 on the foundation paper, (1-5 I think) then that could be a B in old money. The higher end of a grade 5 is equivalent to the lower end of a B, at least for this year. Employers won't 'get it' for years, is my worry. Teachers at the school I work in don't get the equivalency yet.
Reading the government explanations, the bottom of a grade 4 is tied to (i.e. worth the same as) the bottom of a grade C. The bottom of a grade 7 is the same as the bottom of a grade A. A grade 9 is roughly the top third of an A*. So a grade 5 is worth about the same as the top third of a C and the bottom third of a B. A grade 6 is worth about the same as the top two thirds of a B.
The lower grades only have 3 divisions instead of 4 so D, E, F and G are divided up between 3, 2 and 1.
The new standard to aiming for will be a 5, higher than a grade C.
My poor DS3 is in Y10 so will be hit with these new GCSEs for most subjects, not just English and Maths. At least for the first year the proportions of DC getting the grades as explained above will be set to the same proportions as last year. So if say 60% of DC nationally got a C or above in English or maths last year 2016, 60% will get a 4 or above this school year 2017. The same with the A grade/grade 7 boundary. It's tied to last year's proportions for thus next year.