75*3.4 plus (75 * 0.4644 * 24 * 30)
Your demand charge is easy as you have constant demand. Demmand charges are really for traditional businesses which need more whatts during certain hours. Your 24/7 and can just multiply and get them.
This is largely correct. In practice your demand needs to be scaled up to account for losses after the meter in conductors and transformers. This will add 5-10%. Plus your actual usage will increase with temperature so expect an additional 5-10% for your peak loading versus the average.
You also will need to budget for your cooling load. 75 kW of miners cannot be passively cooled, so you will have to account for the fans and A/C required to keep the space in temperature control.
Free advice. Take 1/2 of what you think you will invest and just put it in bitcoins. Use the other half for your buildout. When you have things under control at half capacity and understand the profitability and risks you can make a decision about building out to full capacity.
I partially agree with your cooling statement and partially disagree. You don't need AC to cool them but you do need a lot of air movement to get the heat away. I've got around 96 KW going right now in Texas with only large fans removing air from the room to outside my building and it works fine.