I have a 60x60 building that I will be mounting 12 400w MH lights in. They are low bay lights with acrylic reflectors, mounted at about 14 to 15 ft.
I have hung one temporarily to see what kind of light spread I have, and I think the 12 will just do it for me.
High bay lights do not spread the light as much, it stays more focused directly below the fixture. Many lights like this have adjustable reflectors, that you can raise or lower. I think raising the reflector will help spread the light, so if it is adjustable, consider this.
If you space them evenly (and you should unless some areas of the building are not for use and more for storage, really depends on your layour and use) don't forget that the ones closest to the wall, will be half the distance to the wall, as the distance between the lights. For example, across the 40 ft way, you put three lights, each 13.3 ft apart, and the two closest to the walls will be 6.6 ft from the wall. 6.6 + 13.3 + 13.3 + 6.6 = @40ft..
The 60 ft way you put three lights, each 20ft apart, and the end ones are 10 ft from the walls. 10 + 20 + 20 + 10 =60ft essentially the + sign is a light in the examples. I think you will need all 10, or at least 9 you might retain the tenth for a spare, or install it in a high work area, such as over a corner where you have workbenches, or an area where you always work.
If you had twelve lights you could do an almost even pattern, having each light cover a nearly square 15 x 13.3 area. As it is, with nine lights you would be covering a 20 x 13.3 area, not nearly as good. The bottom of the low bay I hung as a test is 14 ft 6 in from the floor, the spread of light seems usable out to about 15 ft so I will have some areas that are darker than I would care for. Not sure what a high bay would look like, but it won't be as good I'm sure.
You don't mention the voltage. at 400w MH lights draw 2 amps at 240v and 4 amps at 120v. If these are multi tap Consider using the 240v tap but you will need to use a double pole-handle tied circuit breaker and a double pole switch so it will switch off both of the hot feeds (no neutral on 240v). The NEC says you cannot exceed 80% of circuit capacity for lighting (which is considered a continous use item) (and limiting to 80% is always a good practice on any circuit) and as such, a 20 amp breaker can handle four of these lights at 120v, at 240v you can put eight lights on one circuit, thats max. This is alot of current draw. While 10 400 watt lights would seem to draw 4000 watts, the actual draw is 4800 watts, since this is an induction circuit like a flouescent one or a motor circuit, and power factor comes into play. USE ACTUAL AMPS FOUND ON THE LIGHTS DATA PLATE FOR ALL CALCULATIONS , do not do the math using bulb wattage, you could overload the circuit.
In my case I'm using three circuits so I can selectively switch the lights on, that is alot of current and you won't like seeing that meter spinning fast all the time.
Charles
