Dermaroller: How deep do I have to go?

Just picked up two dermarollers. One that is 0.5mm, and one that is 1.5mm. 0.5mm seems fine, peach-fuzz length, I’m fine with it. 1.5mm seems like it is going to puncture through to my cortex

Logic would dictate a depth of about 1mm as adequate as the scalp epidermis is only 0.5mm thick so needles need to go down to a depth of at least 1mm just below it to where the stem cells can get activated. Use minoxidil with it as it helps.