It's pretty easy. You just have to figure out the divisors for a multiplier assuming an HTT of 200. For instance, let's look at 11.5.

First, figure the theoretical product of the baseline HTT and multiplier:
(200 HTT * 11.5x) = 2300MHz Clock

Next, figure out what that clock speed divided by the (real) RAM frequency is, and round up. Take note that DDR2 speeds are marketed as double what they really are. This makes 400 = 200, 533 = 267, 667 = 333, and 800 = 400.

So for the 400MHz setting, you'd do: (200 HTT * 11.5) / 200 = 11.5

Round up and you get 12. So, with an 11.5x multiplier and RAM set to 400 at an HTT of 200MHz, you'd have an actual memory frequency of 192 (384) MHz.

For the 667 setting at 11.5x and 207MHz HTT for example, you'd have:

Ceiling((200 * 11.5) / 333) = 7

(207 * 11.5) / 7 = 340 (680) MHz