Quote Ref: From the 757. OM VOL 2 13.20.10 (the old version)
"The Control Wheels and the Speedbrake Lever send signals to the spoiler controller. The spoiler controller combines the signals from the Control Wheels and the Speedbrake Lever, determines the required spoiler panel deflection, and then signals the soiler panel actuators to operate."
This is the only FBW system on the 757 (i think). For specific deflection angles and combinations I would suggest you try and get a look a the Airplane Maintenance Manual (AMM) . Specifically looking at Flt Cntrl Checks as any testing regime for the spoilers will indicate what deflections are expected for control wheel input etc...
From experience in Malaga or any other suitably Mirrored terminal building I would say the Control wheel input is more like 25 degrees before the roll spoilers are deployed....but thats just a estimate.