Willingness to be critiqued? Self-examination and scrupulous quantities of doubt?

Both of these will help. A lot.

"Actual good" is a fuzzy concept

True. One could go with "that which causes the greatest happiness", but one shouldn't be putting mood-controlling chemicals in the water. One could go with "that which best protects human life", but one shouldn't put humanity into a (very safe) zoo where nothing dangerous or interesting can ever happen to them.

This is therefore a major problem for someone actually trying to be a benevolent leader - how to go about it?

The question I would ask is how you tell the difference between actually achieving the manifestation of your values and only making a big show of it

I'd suggest having some metric by which your values can be measured, and measuring it on a regular basis. For example, if you think that a benevolent leader would do best by reducing crime, then you can measure that by tracking crime statistics.