LOOKUP to return closest match

I am working with an Excel spreadsheet to return a temperature in degrees Celsius when an output voltage for a thermocoupler is entered into a cell. I have two columns of data: the first lists temperatures, the second lists the theromcoupler readings.
I have created a lookup cell in which one can enter an output voltage and then have the temperature returned in the next cell. My concern is that the Lookup function reads the list vertically, and provides a temperature that is equal to or less than a value, but this is not necessarily the best (closest) value. (ie. A reading of 0.516 will return 12 degrees, although a reading of 0.517 is 13 degrees. To be more accurate, I would like LOOKUP to return the closest temperature in all cases, whether that temperature be higher or lower.) I've played around with vLOOKUP, IF, and am having no success.
What must I do to return a better match?
Any assistance would be very much appreciated.

I am not sure this will help, I got the basis of this sheet from someone on this website.

If you enter your known voltages into A and known temperatures into B then you can have the values you will want to find in d.
the spreadhseet then interpolates between the values?

any good?