Mining: Memory clock speed theory I am mining on Radeon HD 7850 2G

I increased GPU clock speed to max 1050 available in AMD overdrive.
and then I started to increase memory clock speed. 
Mining performance is dropping when memory clock speed is higher then 1250.

1300 - 327
1290 - 320
1280 - 313
1270 - 310
1260 - 307
1255 - 341
1250 - 361
1245 - 361
1240 - 360


Does clock speeds must be multiple? 

Is there some formula that shows the best values for mining?
My 7850 has very similar characteristics. 1250Mhz works the best. I doubt there is a formula as my 7870 is much happier with 1500Mhz. Remember that video memory has error correction. Sometimes is can run quite high with lots of errors which will reduce your performance.