I thought the max derate was 25%.
This is the maximum allowed thrust reduction
Off the engine RATING, we can operate engines with two fixed derate ratings, DER1 @ -10% and DER2 @ -20%.
By applying the two types of thrust reductions, you can therefore end up operating a light B744 using a 45% thrust reduction.
I didnt mention DER2 for the B737 as i dont know if they have the 2nd option.
Mutt.