I decrease engine map, decrease brake pressure and if possible, soften suspension. Higher wings.. Looser power/pump locks for easier exits, tighter coast lock to stabilize the rear during braking. Less rear toe in and possibly decrease from front too, this is tough since toe is so wonderful tire management tool but i think less negative in both ends should stabilize the car. We are using high amounts of negative toe in the rear which is unusual configuration for most cars, completely opposite to road car. In very wet conditions you can even try to use positive toe in or put it very close to zero but then the car doesn't turn in to corners like it should during dry conditions. What else... When doing dry/wet setup i only decrease brake pressure, engine map and locks, possibly more wings if it really needs.
One click in the front wing is roughly 4 clicks in the rear. One click to the rearwing was less than 1km/h in top speed so moving balance is easy for wet, just one or two clicks should do it. I did some straightline testing last season so i'm relying on my memory, it is close to those figures.