Not a lot of people seemed to have noticed this so I will bring it to your attention here. As of PanelsUI 0.6.1 beta (Feb 16th), Terrestrial has included a maths evaluation function, $eval.

Valid operators are + – / * @(modulus) {} (instead of parenthesis)

Here are some simple examples:
$eval(1+2*3), outputs 7 – multiplication performed first.
$eval({1+2}/3), outputs 1 – example of parenthesis

For a progress bar example of width 100:
$eval({%playback_time_seconds%*100}/%length_seconds%)