How to make break values which are outside of the value range?

I want to compare mean temperatures for three future periods (2011-2040, 2041-2070, 2071-2100) with the measures mean temperatures of 1971-2000. My resulting shapefiles will show the difference between the simulated and measured temperatures for each period.

I want to have a classification from -1°C. to +3°C. So I would like to have manual break values like -1,0, -0,8, -0,6 and so on. But how can I realize it? For example The difference between 1971-2000 and 2011-2040 is found within a range of -0,69 and +1,88. So the the "coldest" class begins at -0,69°C.

How can I create classes which are outside of the range of the values (for example -1,0 - -0,8)? When I try to edit the break values manually, it automatically jumps back to the coldest existing value (-0,69).