Depending on the brand, but it seems like for most it's double the body weight.

Say you are 200lbs, and you set the slider to 200lbs. That's a 0.2mg/kg dose (the preventive). Setting it to 400lbs should get you to the 0.4mg dose. Note the FLCCC advises even going up to 0.6mg for very high risk, or if you start taking IVM late into the infection.

There was a great spreadsheet someone in /r/ivermectin made. You put in your dose you want, your weight and it'll tell you how many lbs to set the plunger to. Or how many pills of whatever dosage to take. The sub reddit is quarentined but I saved it at home. I'll try to remember to upload it to catbox tonight and start sharing it on here.