I am using "raster calculator" tool of the saga in my model. The tool correctly executes raster operation like "a * 2" in value formula option but the same expression is not executed pre-calculated value formula option.
The syntax for the saga raster calculator always uses "a" as the name of the raster, regardless of what input raster choice you make.
Also, when using "pre calculated value" as the input, you must enter a QGIS expression which returns a string representating the SAGA raster calculator expression...! E.g
'a * ' || @some_input_parameter
Depending on the parameter value, this would evaluate to
'a * 5'
Which is a valid string input for the saga raster calculator