I like 50. 60 is too much and 40 is too low.

I use 50% on our central air with a humidistat controlled variable speed blower. Since it is only effective when the system calls for cooling, we need to overshoot it a bit.

If we had a dedicated dehumidifier, I would set it to 55%.

The manual gives a wide range because in practice the optimum humidity level varies greatly depending on the season and the climate--from as low as 25% to as high as 75%.

Here is a link that might be useful: Fundamentals of Moisture in Houses (Lstiburek & Carmody)