Jeppesen has a 1LE speed loaded for the LJ35, but it doesnt show what the actual speed is.
Did you know that the logic that we use to establish an ETOPS area of operation is based on starting with the Optimum Altitude Cruise speed with a gradual driftdown to 10,000 feet and the 1LE cruise speed, so that means that we start with about 500 KTAS and end with 375 KTAS, the area of operation is therefore much greater than if we utilized 375 KTAS * Approved time, this would have resulted in 750 nms for 120 mins, but using the other method we get 840 nms.
In your case, i doubt that you will get manufacturer support, so take an average LRC/weight combination for 10,000 feet, multiply it by 180 mins and use this as your defined Area of Operation.
Good Luck.
Mutt