Your overall setup isn't to bad. As long as the eggs arn't in direct contact with the substrate, then it's not possible to have too much humidity.
I can see you're trying the "over water" method. This would have to be the best and safest method around.
Just a tip (incase you're not aware already), add some substrate to the water, e.g vermiculite, perlite (the end result should be very wet/runny substrate, not water with bits of floating substrate in it). This works exactly the same as using 100% water, except it stops the water from splashing around to much and wetting the eggs when moving the tub.
In regards to your heatcord setup, I would strongly recommend adding some sort of spacer between the tub and the heatcord (you could use PVC pipe cut to little rings). The idea is to heat the contents of the tub with the air temperature, if the tub is directly on the cord, then it will generate too much heat at the base of the tub (acting more like a heat matt), which will cause condensation to gather on the lid of the tub (because of the temperature difference between the top and bottom of the tub), which can cause water to drop on the eggs. This can be fatal for the eggs.
As well as adding a spacer between the cord and the tub. If you run some of the cord up the sides of the incubator, it will help distribute the heat evenly and should stop the lid from gathering condensation.