I had the whole shebang typed out, with total feed/costs/cog/vog everything, just for my own enjoyment and to compare what others were coming up with. I lost it. iirc, the value of gain was somewhere in the mid 80's. Feed was a bunch, I don't remember exactly something like 60 tons or something like that, beginning at 550 lb calves. If I figured wrong I'll be very embarassed, and if I figured correctly, astonished at the amount it would take. I don't deal with these kind of numbers, so it was an eye opener for me. Maybe someone who deals with this kind of volume will jump in.
I do see a problem with your math so far. You didn't say what they weighed exactly, but at 500 even, 3% is 15 pounds. If they gain at 6 to 1, in 30 days that's adding 75 lbs. Beginning the last 65 days at 575 lbs, at 3%, that's 17.25 lbs/day. So, I would say,
15 lbs/day(70)(30) is 15.75 tons
17.25 lbs/day(70)(65) is 39.24 tons
at the very minimum, with the above given information. I am curious as to what you will be feeding in each segment, and how the price is so far apart? I am wondering if that is normal to start calves at 3 percent, and continue at that same rate, adjusting the energy or protein within that 3 percent throughout the backgrounding period?
I keep my calves a while, but I am not feeding that much feed. Not even close. But, I am not trying to get them too fleshy as I will not be retaining ownership of them yet, and buyers are wanting a certain amount of compensatory gain when they buy them. If I was retaining them I suppose it may pay to feed them more, as I would be the benefactor of the increased feed bill during backgrounding. I have heard what I am trying to do referred to as growing frame.
If you don't mind, what are the particulars of your situation? Custom backgrounding? If your own calves, will you be retaining ownership?