Yeah rectangle ones are not perfect, and some rectangle ones are even larger than others even though they're all advertised as 2x3x4. When they're larger, it's hard to seat them into the switch perfectly straight.
Anyways, if you really want accurate brightness, trial and error is the only way to go. Advertised brightness (mcd) specs on mouser and digikey won't turn out to be what you expect. When I first bought them, I went by specs and turns out same color, same forward voltage, and same brightness turned out to have different brightness using the same resistor value. I never go by those mcd numbers anymore. Resistors and LEDs are cheap enough where you can just buy a bunch to experiment with.