From long-ago experience Diafine is a particular developer for a particular purpose: pushing. Accordingly, it limits the contrast of highlights. The resulting images, from what I recall, have a characteristic look of compensating developer: the highlights are dull. If you want to mix your own, why not D76, seldom surpassed by other fancy formulations. Use 1+1 one-shot. 10min @20°C as a starting point; then define your preferred time, probably in the range 10-12min.