One could do an engineering calculation, but it would be complex and only approximately accurate. There are too many variables here. These variables include the time of year (where the sun is in the sky), the time of day when you want to measure the temp, the wind level, the actual transmissivity of the glazing, how well the greenhouse is sealed (i.e., amount of air leakage), what the frame is constructed of and its heat capacity, what the floor is made of and its heat capacity, and so on. The easy answer is to keep a record of exterior and interior temperatures, and weather conditions, to get an idea for your particular setup.

I have an electronic thermometer in my greenhouse that transmits wirelessly into my house so I can keep track of temperature. But I have to say that I am frequently surprised by what the temperature is, compared to my best guess given weather conditions. As I sit here typing at 10:30 in the morning in the Pacific NW, it's 47F outside, 64F in my greenhouse, and totally clouded over.

Yesterday it was cloudy here, in the low 60's, and 84 degrees in the greenhouse. One day this winter, it was sunny, around 30 degrees outside, and around 100 degrees in the greenhouse. It just all depends on how much sun you get, and when...and the temperature drops fairly quickly for me when the sun goes behind the clouds on a colder day. I second the wireless remote thermometer - it's really the only way to get a good "feel" of what the temperature swings are like out there.