In my Octopus account it says that there is data available for every home in the country based on historical usage and weather patterns that predicts what your usage will be.
Obviously if you moved house and had very different usage levels to the previous occupants, made deliberate attempts to reduce your consumption or changed something about the heating, hot water or cooking appliances then the data will no longer apply, but otherwise they should be making predictions based on your actual usage.
From your point of view, the most important thing to check is that different pricing (fixes vs variable) are based on the same amount of energy. Also that its comparable to your actual bills (not direct debit amounts) for the last year or two.
DD amounts don't always line up with usage very well as you could be paying off debt or have previously used up a credit. Or there could have been a time lag between the unit price going up and the DD amount catching up.
But in order to decide if that fix is worth it, you need to compare it to the standard rate that you're on now and are also likely to be on from October (expected to be 65% more than the current price).