Since through using KXStudio's Cadence or directly in the .jackdrc file you can over-ride JACK's drop-down menu selections and type-in your own custom frames or buffer entry, you can enter the numbers below and thus achieve an even multiple of 1ms (that is, a whole-numbered latency figure) for use with USB interfaces which require that.
I created this spread-sheet that produced the numbers that generated integer-multiples of 1ms for both periods (2 & 3), suggesting what may be possible ideal candidates for USB interfaces. I couldn't get the numbers to line-up under the labels, but you can figure out which are which.
Are those "oddball" frame [or buffer] values really going to work for you? Try them and please report back!
The issue of being integer-multiples of 1ms does not matter to non-USB devices.
Many thanks to Paul Davis and Stephane Letz for JACK and to AutoStatic at linuxmusicians.org for alerting me to the USB forumula used above.
This article was written by Terry Leigh Britton and may be redistributed in terms of CC by-nc-sa.
Many of the effects and amp simulators in Guitarix depend on a library which expects your frames/period to be a power of two (16, 32, 64, 128, etc), and large parts of Guitarix fail to work properly if this is not the case. This can easily be resolved by choosing periods=3, which enables most of the "conventional" 2^n frames/period values. In fact, if you choose periods=2, it will be impossible to get Guitarix to work properly while also having a latency which is an integral number of ms.