Everything is a best guess estimate and there will be a lot of factors that can affect it. Cable gauge, actual length, number of joints, amount above ground, background noise and proximity to industrial premises, number of ADSL and VDSL customers in cable, position of your pair in the larger cable ...
I am currently 450m from the cabinet (actual) and am forecast to receive 57/19 - the modem however gives attainable rate figures of around 63/24 - which does vary slightly.
So, if everything is "average" and yours is typical then the checker estimate will be in the right ball park.
M H C
taurus excreta cerebrum vincit