The "exact ratio" that constitutes the best mixed spectrum light has yet to be widely agreed upon! But even your 400 watts of MH between two 1,000 watters of HPS is still only about 20% MH overall.
Many of the "Single Combination" supporters (people that favor using the same light source for vegging and flowering) favor a "dual spectrum that is closer to being balanced. For example, 50% MH to 50% HPS - or - 40% MH to 60% HPS. Keep in mind that either of these two ratios can be had by combining MH and HPS units of various wattages - or by using dedicated "Dual Spectrum" bulbs.
The other "common" approach is the "1/3 - 2/3 Ratio" (my personnal favorite!). This method requires changing the "Spectrum Ratio" at flowering time. For example, veg with MH @ about twice the wattage, and, HPS @ about half the wattage (2 MH units and 1 HPS unit, of the same wattage).
Then, when it's time to flower, you switch to 2 HPS units and 1 MH unit, of the same wattage. This way you keep the ratio about the same, 1/3 - 2/3, but you change the spectrum. At this point the extra HPS also adds a little "bonus" for the flowering process!