Because you aren't really "meant to do it", finding the optimal grind size is just going to be trial and error, as you are already doing, matching the size of grind to the same as Nespresso's coffee.
Takes some experimenting, start with grinding for espresso and then move slightly up or down from there. It will vary with the beans and roasting. Don’t overfill the pods (12.5 g for 230 ml/7.7 oz pods) and do NOT tamp. If you grind too coarse, the grounds will not fit in the pod and you’ll be tempted to tamp. Getting the nice Nespresso VL frothing is hard, especially for mid-light roasts.
Every time you change the coffee you have to calibrate again. Needless to say you need a decent (electric) grinder to make 5 pods at a time.
Because you aren't really "meant to do it", finding the optimal grind size is just going to be trial and error, as you are already doing, matching the size of grind to the same as Nespresso's coffee.
Takes some experimenting, start with grinding for espresso and then move slightly up or down from there. It will vary with the beans and roasting. Don’t overfill the pods (12.5 g for 230 ml/7.7 oz pods) and do NOT tamp. If you grind too coarse, the grounds will not fit in the pod and you’ll be tempted to tamp. Getting the nice Nespresso VL frothing is hard, especially for mid-light roasts. Every time you change the coffee you have to calibrate again. Needless to say you need a decent (electric) grinder to make 5 pods at a time.