As others have noted, the STA is not a factor in figuring the reach difference. It only affects the amount of seat post offset you might require.

If you're using no spacers and a -17 stem, you need less stack height. If the smaller frame has 7mm less reach, you'll need a 10mm longer stem, assuming that the same bar reach is used.

One problem that can arise is the one piece bar and stem setups limit the stem angle and bar reach selection.