Drag a double type onto the canvas and assign a double value to it to use it as a variable. Use the SetValue to change the value further below in your process.
A double holds signed IEEE 64-bit (8-byte) double-precision floating-point numbers that range in value from -1.79769313486231570E+308 through -4.94065645841246544E-324. The Double data type provides the largest and smallest possible magnitudes for a number. The default value of Double is 0.