You add one bit of checksum for every 32 bits of entropy.

This way the sum of bits will always be a multiple of 11:

• with an entropy of 128 bits (4 x 32) you add 4 bits of entropy = 132 bits (12 x 11)
• with an entropy of 160 bits (5 x 32) you add 5 bits of entropy = 165 bits (15 x 11)
• with an entropy of 192 bits (6 x 32) you add 6 bits of entropy = 198 bits (18 x 11)
• with an entropy of 224 bits (7 x 32) you add 7 bits of entropy = 231 bits (21 x 11)
• with an entropy of 256 bits (8 x 32) you add 8 bits of entropy = 264 bits (24 x 11)