Around 40 to 50mm should should be about right. Although measure the arch gap with standard ride height, then take the standard suspension off, stand the coilovers next to them and then wind them down so they meet your previous measurement and then refit, drop the car to see how it sits and adjust by eye as necessary.
You'll want to bear in mind that the suspension will settle after a bit and sit a little lower so you may want to factor that in, start about 5-10mm higher than you really want so you don't get scrubbing and expect to readjust a couple of hundred miles later.