No, you cannot take surface temps without either a Temp-gun or thermometer/hygrom. with a probe, it sounds like you`re measuring the air, and if it`s 40c directly under the heat bulb it may well be within the recommended range, but you need to know precisely. EDIT: You say you`ll order one for the new tank, but you need one for the current one asap.
Is this similar to what you`re using just now?

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