First off, how are you taking the temp readings on the UTH? Using a temp gun directly on the cage floor? That is the only true way. Also like I mentioned in another thread, a UTH isn't going to heat the air. If you are using a tank, I recommend throwing it out and getting a PVC cage. Glass tanks are fine for a QT cage but I wouldn't use it as a permanent home. I would get a lamp dimmer from Home Depot or Lowes for the CHE and dial it in until you get the appropriate cool side temp. The UTH will provide the belly heat assuming you get it set right.
As for how forgiving, depends on the snake. Some are more, some are less. I'm guessing this is a baby. Babies dehydrate a lot faster than adults so I would get your stuff squared away. Having something slightly off temporarily is fine but you don't want to go long term.
You don't need a hydrometer. Just get a digital thermometer with humidity and temp. I use the ones by acurite from Walmart. They aren't exact but are within 5% or so. That's plenty accurate for the snake. I keep my boas and retic around 85% humidity and my BP around 75-80%. You just don't want condensation all over.
And you MUST have a t-stat for any belly heat and really should have a t-stat for any heat source but you can generally get away with a lamp dimmer for a CHE outside the cage just for ambient temps. You want a good T-stat. I use Herpstats and have for years. Just ordered a Herpstat 6 today actually. Vivarium Electronics are good too but don't have as many safeties and bells and whistles as Herpstats but are cheaper. Ley is you want safeties on them like turning off all power on a failure or turning off power if the temp gets too hot.