Radiant cooling accounts for something like 7-10% of your heat loss. Tarps/flys will generally stop most of this. Assuming radiation occurs in all directions, you will save about 5% by using a tarp over you, ignoring the ground.
Convection depends on the air circulation under a tarp/fly. This is HIGHLY variable bit I figure about another 10%. Sometimes, with no wind or breeze, this is close to none. Sometimes, the wind is soo heavy you can easly add 10F or more.
A single walled tent/tarp supplys no insulation. But a double walled tent will add about an R1 to a tent, assuming a 1" air space all around. Not that for some meshes, non-noseeumm meshes, this can be a bit less, but it helps. Usually I figure 2-3 degrees per R digit, but this is by guess and by gosh. It is usually more, but this does not take into account open edges on a tent.
Exhaled heat trapped (another 2-3%,) moisture/condensation (another 1%,) and general air insulation of smaller tents (~32sqft, ~40" high) accounts for another 4-5%.
The overall heat loss to the ground is highly variable, too. Rocks, hard packed earth, and ice will be colder than snow, forest duff, or lawn, for example.
Overall, I figure about 10-15F increase due to the tent/tarp. It could be more on a still night. Best case is about 25-30F, assuming ideal conditions. But, you cannot allow for best case.