Jim,
Sorry, but the length of the coax is important. You
have another option (per the instructions), add another
12 feet to the 77 feet you already have.
The length of coax is specified for a purpose, it makes
up part of a circuit which does impedance transformation, and
if you change the 'value' of a circuit's component, you
're-tune' the circuit. The length is fairly critical. Maybe
not to the 'milimeter' level of accuracy, but certainly to the
'foot' level...
'Doc

PS - Are you sure the instructions said 45 feet or 90 feet?