The problem with very long runs is that is very hard to guesstimate from a far. To begin with, LNBs do not have a set output, I have sen LNBs with as low as -19dBms and as high as -29dBms. this is a huge margin! the second is that is unknown what is the lost of the cable being used, according to the link below, that particular cable looses 6.5dBms per 100 feet. So assuming the LNB output is -30dBm, and a run of 250 feet, plus a 2 way splitter, that should equal to
2 way 2.5dBm
250 RG6 16.5dBm
fittings 2dBm
Total loss 21dBm
a receiver threshold is from -30dBm to -60dBm. in this case there is about 9dBm of "playroom" of course this is all "lab" numbers and changes in real life