75-80% of what? By its self means absolutely nothing.
It could be too high or too low depending on how much it is a % of. RI is caused by a bacteria, a virus, or a fungus. Aside from a virus they need humidity and poor air flow to proliferate.
Today it is 95% RH outside, it is -5ºC. That is 3 gm/m3 absolute. At 26ºC (80ºF) 3 gm/m3 absolute is only 13% RH. A % will vary if the amount it is a % of varies. 10% of 1$ is not vary much money 10% of a million dollars is quite a lot. Saying I have 10% means nothing with out knowing what you have 10% of.
My rack is exactly the same, pvcx, not wood tho. You keep your accurite inside your tubs?
if you are certain your ambients are that high it is possible you have a bigger issue than just humidity. It is certain you need 15.5gm/m3 absolute for a royal you have a good shed and your ambient temp of 80º has a saturation of 25.3 gm/m3 so 75% of 25.3 is close to 19 gm/m3. That is quite a lot more than you need and is starting at the point where you will likely increase the bacteria load.