Understand the distinction
An estimate can be helpful for planning when its assumptions remain visible. Trouble begins when an estimated operating time or consumption value is later presented as a measurement. The distinction matters even if the estimate uses a reasonable formula.
Put it in your home notes
Label each entry as measured, supplied, estimated or unknown. Retain the date and method. When better information arrives, add a revised entry rather than erasing the original assumption. This gives a later reader a clear account of what the numbers can and cannot support.