How to calculate SCLK frequency?

Hello I'm relatively new to SPI so this is likely a simple question. I am wondering how one calculates the clock rate on the SCLK pin when using SPI. According to the PICBASIC manual the clock is approximately 50kHz and can be slowed using DEFINE SHIFT_PAUSEUS x. How do I determine how many ms I need to slow the clock to achieve different clock rates. The product I'm trying to interface to only supports a max 40kHz clock. I have been able to get the two to "talk" but I really need to know the approximate rate.

Thanks.

G.