Boots £10 baby thermometers are not reliable - using 2 at the same time in the Mini they came up with a 1C difference. One thermometer was 1 year old, the other one brand new. So that one is on the not-recommended list.
I used the Heston Blumenthal Precise digital meat thermometer, which was always fast and seemed more accurate than any of the others but is now no longer available in that model, so I'm now considering a Brinsea certified calibrated incubator thermometer though it's a bit pricey.
Remember to measure the air temperature at the height of the top of an egg, and don't let the thermometer lean against a shell.
You're so right in double checking the temperature. I always keep mine on 37.5-38C (thermometers - they are never in perfect agreement) and the hatching rates with our own eggs have always been 90% or 100% except once: the first time a secondhand Octagon was used and the temperature was 1.5-2C too low. The temperature hadn't been double checked for some reason and it remained too low for the whole incubation period, consequently the hatch rate halved and several chicks had leg problems to the extent that they needed culling. We could have kicked ourselves
You'll be safe with a 0.5C difference, and hatches will be a bit later or earlier with a 1C difference, but larger long term differences will have a significant detrimental effect. You'll be fine with 38C, though.
Will you be candling in a few days?