A running bond pattern is normally 50% (plus grout space) ...is that want you needed?
not sure what you mean by bowing tiles?...tiles and the substrate they are being set on should never flex or bow
Rectangular porcelain and ceramic tiles tend to flex or bow when set at a 50% bond pattern.
Are you implying that the grout lines are not straight?...I like to use stiffer tile spacers to ensure they do not get squished and throw off the pattern.
I also will use small bits of painters tape to provide a bit of tile to tile tension against the spacers...this results in perfect grout lines.