At the moment you can only set up for either inters or wets but not both which I think is a serious handicap if you're unable to attend a race and number of laps before changing back to dry tyres is a little too random.
In another game (and I can't remember which one as I only tried it once) the wetness was a percentage instead of mm. iirc you could set a percentage to go from dry to inter, another to go from inter to wet and a 'window' to prevent unnecessary pitstops if the weather changed at the percentage set.
So for example, 20% inter/ 50% wet / 5% window that way if the track dries you'd go from wet to inter at 45% and inter to dry at 15%