It all depends
The placement of the snatch block is highly dependent on wind conditions and the type of head sail used. Considering just the wind conditions: heavy winds demand a flattened sail while light winds demand a fuller sail. Thus, in light airs and for the 25 (assuming that the winch location is roughly the same as mine), the block should be slightly ahead of or even with the cabin bulkhead. As the air builds, the block must be moved back. Heavy airs demands a flat sheet and as far back as you can move the block the better. This is basically how is set up for my 155 Genoa. The type of sail also dictates the placement of the shocks. My description above is for the 155 Genoa. The 130% Genoa will demand different settings (primarily on the heavy wind side / where the blocks will move forward slightly as compared to the 155%). The 110% or a jib will be different yet. Play with your settings ... your speed, the pressure on the tiller and the heel of the boat will be your best indicators of correct block placement.