As I said, I was a little surprised myself as I too thought that the 'Low' selection trimmed back more in the region of 30 Deg C, but having dug the RR RB199 course notes out, under 'Dry Engine Control (GR1-MECU)' it gives a temperature change of 24 Deg C for both 'Combat' and 'Low' selection which is governed by the TBT datum signal versus 'Actual TBT' passing through error units, with the 'Lowest Fuel Wins' signal acting upon the LPC solenoid to reduce engine fuel flow
How utterly, utterly sad am I?!?
Flipflopman