Hi UncleUB,
I think you are asking an impossible question in a highly complex subject. There are many different forms of direct and indirect REIN / RFI and hopefully ezzer can provide some examples of his experiencs. From the radio transmission viewpoint there will be some form of square law of distance and radiated power. Antenna physics regarding aerial lengths and angular alignment will probably be involved.
Then there is the actual method of absorption by your equipment and line. The one I solved by accident recently happened to be caused by PIR Lights on the next but one's garage which affected the neighbour's lines and infected "our" circuit via cross talk in the riser cable from a road joint pit. Last year I came across another crosstalk problem which was caused by an old analogue wireless phone only when it was being used in a house three roads away from the affected house, but all fed from the same cabinet and D side cable. The permutations are endless.
How to solve the problem might be a more profitable approach. (Again HELP ezzer.) You could try a quiet line test at the expected time to see if anything can be gleaned that way. You could try the AM wireless method on several occasions with the radio being rotated to try to find the direction. You could run Routerstats as fast as you dare over the time periods. An audio recorder or an oscilloscope attached to the line or to a wire coiled around your line might be another approach. Re-siteing the modem or using different cables and mains filtering could be tried. Experimenting with an RF3 filter might hide the problem, but at the risk of reducing overall performance. etc. etc.
I hope these ramblings might trigger thoughts from others.
Kind regards,
Walter