I don't use a tach to set my gassers. It's all by ear...... Set the low for good idle and transition, and rich enough where it won't lean out in a harrier or rolling harrier. I set the high speed to where I can TR for a minute or so, then go full throttle straight up till I can't hardly see it anymore. If it doesn't lean out, you're ok. Also if you plane is propped right, do some full throttle horizontal runs and let it wind up and unload, land, and pull the plug. As long as the plug is tan and not white, you're good to go...... If it's white, you're too lean and need to richen the hs needle a bit.