There was an interesting thread "jack dsp load calculation" on http://lists.jackaudio.org/listinfo.cgi/jack-devel-jackaudio.org, opened by Robin on Wed, 23 Dec 2015 16:35:33 +0100. Unfortunately it's not available in the archive: http://lists.jackaudio.org/private.cgi/jack-devel-jackaudio.org/2015-December/date.html :(.