The Thrust/N1 curve is logaritmic.
Take the classic 737 as an example, CFM56-3 engine.
If you increase N1 from 80-90%,
you will add about 5000 lbf thrust.
If you increase N1 from 30-40%
you will add about 1000 lbf thrust.
(ISA, sealevel etc)
Cheers,
M