It means that max thrust will be reduced above 30 celsius, but the important information is the value of the Flat rated max thrust...
Ah, not especially.
Why make things more difficult than they need to be?
For example, the L1011 with RB.211-524B02 engines has, as standard at many airlines, runway analysis takeoff charts supplied that permit ops at sea level to 54C (ISA+39), so the assumed temperature method of calculating MTOW is very straightforward....even without the fancy FMC's of today.
Piece of cake.