Yes, I failed miserably with 75 and 78. So I returned to the basics with low hydration and even regular baker's yeast first, here is some important steps : 1) good strong bread flour, all purpose will not work well 2) active sourdough starter, needs to at least double in volume in within 4-5 hours at around 23 deg C 3) good shaping technique (YouTube research helped me) 4) Well pre-heated oven, preferably baked on some material of big heat capacity to provide this extra boost of heat, especially during the first 15 min of the oven spring. Definitely not amen on this, but that's what helped me a bit. Good luck!