the % of wetness shown on the hud is informative but can make you think wrongly
from what i understood,
it's the % of wetness of the best driving line (the one you use when the track is dry) on the portion of the track you are on
maybe at the level of your car or for the next 100m, there is nothing to really know that
if you follow the rule that
in between 25% and 60% you can still drive (with care) on racing soft
in between 50% adn 75% on Rain Int
above 70 % on Full Rain
it will be fine
yes i have overlapping rules

i depends of your car, temperature and track surface grip
but as i said you can missjudge the wetness as rain goes more intensive or tracks dries up
because it's the % of wetness of the best dry driving line
let's case that the tracks is drying, if you're not on the best line, even at 0% , you will see water spray behind the car
same if it starts to rain, on the best line you see no water spray at 0%, move a bit left or right and you can see some starting to appear
that's also where it become dull, on some track you don't even notice that the surface is covered by water
that system makes you use the best "dry" driving line on wet surface, something that real racers will laugh loud about
even if every one drive on the left side and the best dry driving line is on the right, it will not starts to dry on the left but on the right ..
GT is not the real rain simulator at all