When I've done B&W reversal (been a while, but I have my notes), I first developed in Dektol 2+1 (two parts stock to one part water) for twelve minutes. I'm pretty sure that's development to completion. I then bleached to completion as well (the bleach removes only developed silver), and the second developer can also be run to completion, since both contrast and overall density are controlled by the first dev. Doing this with early 2000s vintage Tri-X, I got a workable speed of EI 640, but different films will give different results in terms of speed relative to box speed.
Either underexposure or underdevlopment in the first dev will result in dark final slides. if the slide is dark and there's not much separation between lights and darks, it's probably underdeveloped; if the separation is good, but everything is too dark, it's likely underexposed. If you can't be sure which, it's probably both, so start with increasing first development, then adjust exposure in later rolls once you have good separation of values.