The Brake Buddy is designed to work with no power assist in the brake pedal. You can't test it with the engine running because the power brakes will function, over powering the brake pedal. With engine off, you should push the test button enough times (6-8) to bleed off the vacuum assist. I do it to ensure that the pedal clamp is securely in place and to bleed the vacuum. I've tested the BB's sensitivity by positioning it on the passenger seat floor and driving to see how much deceleration is required to trigger it, but you can't test the braking pressure with any vacuum left in the brake booster. Start with the recommended settings and tow it around the block. You could have someone ride in the passenger seat to report if it's braking too hard, but that's the only way.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI