I have quite a large water bowl in there with her but she is 4.5 feet long and weighs about 1.8kg so not too big for her, No I am using a analog Hydrometer (I know there rubbish) i have a digi on the way
If there isn't condensation running down the walls and the substrate is dry (it really should be even though orchid bark etc arrives wet) its almost certain to be fine. It will be very unlikely to ever fall below about 50% without using dehumidifiers. And wont be higher than about 70% without causing condensation. So should always fall into the right range.
I dont monitor humidity, I did, it never really changes so i scrapped wasting batteries in hygrometers. Once you get in the way of things, substrate is dried before it goes into the vivarium, water bowl positioning in the cooler end then you will see it doesn't actually move all that much and you only need to increase humidity when they shed (or not even for that, Duke shed over the weekend and I didnt even know he was blue).
For me 65-70% is perfect. I wouldn't worry too much. I have got exactly the same humidity level in my viv for the last 17 months and my royal is fine. If you put water bowl in the coolest place of the viv it should go down little bit. Or you may get little bit bigger vent holes but I wouldn't touch it to be honest.