change I2C speed WITHOUT modifying library?

[system]

So I guess my question is about the prescaler.

The first answerer above said:

uint32_t prescaler = 4 ^ (TWSR & 0x03);
uint32_t freq = 16000000L / (16 + 2 * TWBR * prescaler);

But freq would be 400 only if prescaler = 1.