As
@touch says, the calculation isn't complex - it's just your annual usage/12. Before April, my DD amount didn't change in 12 months, and has perfectly covered my annual usage to within £12, because I set it to exactly the right amount to cover my usage.
Obviously if you've just moved into a new property you may not have an accurate idea of what your usage is going to be, but you clearly aren't in that position since you're making such a fuss about it.
The algorithm will be based on averaging hundreds (if not thousands) of data points from existing customers, so obviously it's not going to accurately reflect someone with atypical usage such as yourself, but like I said, if you know your usage then just set the DD to a figure
you know will cover your usage. If you can't be bothered to do that then don't complain you aren't paying the right amount.
Except you don't. I don't understand why this is such a difficult concept to grasp. If you use 5,000 units of electricity @ 30p and 10,000 units of gas @ 10p, then you pay for 5,000 units of electricity @ 30p and 10,000 units of gas @ 10p. It doesn't matter* whether you pay by fixed monthly DD, quarterly DD, pay in arrears when you receive the bill, send a cheque to the supplier, deliver a truckload of 1p coins in person or barter for a herd of goats, you still only pay for 5,000 units of electricity @ 30p and 10,000 units of gas @ 10p. You may end up with a credit on your account if the DD is set too high, but that money is still yours, it doesn't just disappear, and you can either request a refund or reduce your DD to below your usage so that the credit gradually reduces. Regardless, you still end up paying the same amount.
* we'll ignore any discounts given for paying by DD for the sake of simplicity.