Hi David
-ve occur because we measure it from the receiver end.
FSPL means free space loss
(FSPL + Rx Antenna Gain − Cable/Connector Loss)=Total Line loss.
EIRP= Tx Power − Cable/Connector Loss + Tx Antenna Gain.
So RX Lev =EIRP- Total Line loss.
As per your example
Rx lev=-EIRP(since we r on the receiver side)-Total Line loss.
hope u got it