GUNSON GASTESTER - ANY GOOD?

Anyone used a Gunson Gastester to set up carbs ready for MOT? They seem really cheap compared to the 'professional' testers so I wonder if they are sufficiently accurate to get emissions, particularly on early carb V8's, below the required MOT levels?

On an mot on my v8, I adjusted the carbs while he was doing the emission test to get it to pass. So I bought one of those. Sat in the cupboard for a year, I went to use it, but it was no where near possible to get a setup reading of 2.5, or whatever the setup number was. I had to take it apart and fiddle with some variable “resistors maybe” to get it to setup.
I started by balancing the carbs with a flow meter, then set the exhaust emission air fuel ratio with a hego sensor and reader that I have, then used the gunson thing to bring the co down a bit. Did that a couple of times and all seams well.