For this purpose you should first set new core and memory frequencies and make them as close to the desired figures as possible
Now look for this section in the registry:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Class\Display\0000\NVidia\NVTweak\Hardware:
(if you have several versions of drivers installed, then you may have to use \0001\, \0002\ etc. instead of \0000\ ). Now copy the content of Krusty key to the neighboring Bart key.
That is all the fuss. Now you may vary the frequencies within the total interval of 50% (i.e. +/- 25%), but the zero point is assigned to the newly introduced values.