Getting the protein level of the flour

To obtain a desired protein level of the flour, two flours need to be mixed: a low protein flour and a high protein flour.

Notation:
Required percentage of low protein flour = 100% x (HPF - DPL)/(HPF - LPF)
Required percentage of high protein flour = 100% x (DPL - LPF)/(HPF - LPF)

Example

Required percentage of low protein flour = 100% x (13.1 - 11.5)/(13.1 - 10.8) = 70%
Required percentage of high protein flour = 100% x (11.5 - 10.8)/(13.1 - 10.8) = 30%

Thus to obtain 1 kg of 11.5% flour we mix 700 gram of the low protein flour with 300 gram of the high protein flour.