It's impossible to use variables in filter's parameters.
For example, Resize could be usefull to have %i_width%*2, but the input field allows numbers only, though there is a button for variables insertion.
The same issues with Pad, Crop, and other filters.
Use of variables in filters
Use of variables in filters
Alexander Sorkin
Re: Use of variables in filters
OMG! This is embarrassing!
Can't believe that passed the radar... Thanks for reporting! 
-steipalr


-steipalr
Re: Use of variables in filters
Thanks!
The next update resolves this issue. It will be released in a couple of days.
-stipal

-stipal