framebuffer & sdram pll

Hi all

I am making a video system with a DE2 and the sopc builder.

Right now, i am trying to use the vip frame buffer, but I have not idea about the clock delay that I have to give to the sdram.

According to the DE2 examples, it should be -3 ns, but it doesn'seem to work properly.

Anyone could (please) tell me how to proceed in order to find the right delay for the sdram clock?

edit: i do not use the NiosII, i don't need (still) a cpu.