Thanks for your reply Pete...
The problem is that currently my software isn't able to add or substract variables. It provides an interface which allows the user to define a variable (specify a name, the offset, the format, a multiplier) and that's all. Then this variable is used to build ARINC429 frames and send them to an avionic device.
To allow the user to do this kind of things (add, substract and so on), I'll have to change a lot of things on my software and it will make it far more complex ! As the software will be used on different avionics devices, I can't "code in hard" that one specific variable is the sum or substraction of two others but I have to provide the user with an interface to do that (so develop a kind of scripting language that he will be able to use or something like that...).
That's why it would have been far easier for me to use directly an offset to access the RADIO_HEIGHT var for exemple. I don't know if such an offset exists or if you did the substraction in fslook...
Regards,
Ed