All you should need is an if statement that says "if button is pressed, then set baseline temp to whatever your temperature variable is"
If you want another challenge to spice up the loveometer you can do what I did. Instead of just taking 1 sensorVal reading, take 5 and use the average of them to calculate the temperature. It was a fun exercise and also made it so my temperature was much more consistent.
You could use 5 sensors but I feel that would be overkill. I just read the sensor 5 times and stored each in a different variable (sensorVal1-5) then averaged those before calculating the temperature.