For a regular sump, 10x the tank size is good. This is the amount of flow coming into the tank, not the pump size. 10 x 90 = 900. The pump will have to be larger than 900 GPH to compensate for head height. Measure how high the water will need to be pumped to flow back into the tank, and check the flow on pumps to see how much flow there is at that height.
For a sump with refugium, you want the flow to be slower, about 4x - 6x the tank size. 4 x 90 = 360, 6 x 90 = 540. Something between 360 - 540 GPH is what you want for a refugium, and again, this is the amount of flow to the top of the tank. It is important to know the head height flow of the pump, because the flow in the sump or refugium will only be as strong as the flow going into the tank. For instance: if it is 5' from the floor to the top of the tank, and a pump is rated at 550 GPH at 5', it is perfect for a refugium. If a pump is rated at 900 GPH at 5', it would be perfect for a sump. I know all this can be confusing, but it is very important when it comes to choosing the proper pump. It's okay to put in a larger pump and (as Imforbis mentioned) install a ball or gate valve on the return side of the pump. This will allow you to fine-tune the flow.