> I've been reading AAs book and looking into his concept of N+x and N-x development of Ilford films.(posted 8311 days ago)I think you're getting the concepts confused; pushing is the intentional underexposure and appropriate (over)development in order to make a printable (but high contrast) neg, which your FP4 example would result in, while your HP5 example is simply going to give normal contrast but at a lower speed because of the developer used.
N+ and N- development otoh is simply adjusting neg contrast to fit the subject brightness range so that the neg can be printed on a "normal" paper grade.
I've found that roughly a 20% development time change is a good starting point, depending on the film. Note that EI doesn't change a whole lot; N-1 needs only 1/3 to 1/2 stop more exposure and N+1 may not need any less exposure to match speedpoint density with normal development. However, you may want to give more exposure for N- to get the shadows up on the straighter part of the curve.
There's no need to get really retentive about this; such minor changes can be handled by using different-contrast papers. There are of course differences but they're subtle.
A very good book on the subject is _Beyond the Zone System_ by Phil Davis.