Yarn Substitution

The pattern I am overthinking is the Nansen Ponchette.  I am making a number of changes starting with a different yarn. Some yarn substitutions are simple, some are more complicated.

The yarn the pattern calls (Berroco Flicker) for has a default gauge of 5 sts & 7 rows to the inch on a size 9.  The yarn I am substituting (Berroco Peruvia) has a gauge of 4.25 sts & 5.5 rows on a size 10.  Not too different, but enough that some adjustments will need to be made.

Just how far off? My pattern calls for 48 stitches to be cast on.  The resulting panel would be 9.6" wide.  With my substitution, 48 sts would be 11.3" wide. My panel would be almost 2" wider than the pattern.

This is where the all important gauge swatch and a little guess work come in to play.  The pattern I am using has a slightly different gauge than the yarn ball AND it is measuring that gauge in pattern.  It wants 48 sts to 7" and 20 rows to 2.75" IN CHARTED PATTERN.

My pre-swatch assumptions:
1) I would need to be a little smaller than the called for yarn if I don't want to change the number of stitches.
2) I tend to knit pretty close to gauge.
3) This is a poncho and fit is not as important as it would be for a more fitted item, however, drape is very important.
Conclusion: I'll try a size 8 needle for my swatch instead of the 9 called for in the pattern.  I don't want to adjust the number of stitches in this pattern if I can get away with it.  The length of the knitting in this pattern (rows) is all indicated with inches, but my row gauge matters because after joining this panel, I have to pick up around both edges to add the top and bottom of the poncho.