Dont know if this helps (Boeing) 25% Max reduction in thrust for the T/O thrust rating used for the takeoff.
For example on Ryanairs 800 there are two thrust derates. The engines are 26K and they have 24K and 22K derates line selectable in the FMC. The 25% applies to the derate used. So 22K derate with a 25% thrust reduction would be a lot, maybe as much as 40% of the engines rated 26K thrust. But because you are using 22K derate its legal. One of the many reasons for derates.
As far as knowing where 25% is the FMC protects for that (Smiths). Also in the Boeing performace Inflight section QRH you can see the max temp that would generate a max 25% reduction for 26/24 and 22K takeoffs. The FMC will accept a higher temp for takeoff but the thrust reduction will not exceed 25%
Best Rgds