Had the pleasure of Mr Sunny Jim today and he happen to have is compression gauge with him so I swiftly checked his gauge against my $60 special and also compared there readings to what the big boyz calculator had to say.

SJ gauge has been calabrated it an instrument shop with a Calibration cert on the gauge

SJ Gauge
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Ando's Gauge ( Bleed of a whisker by time I took pic)
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Big Boyz Calculator
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the readings are nice and close , another variable to readings with comp gauge is starter cranking speed ,that is why you may not see there readings , or  a down or slow  starter will not show as higher readings as say a new one , or lead problems ,, i was taught , open aircleaner , full open throttle , 5 rotations of eng  no more or less,, if you do 3 rotaions on one cylinder ,and 5 on rear you will see different readings on gauge ,  , do this  3 times and work  average of what they are , then do other cyclinder to then compare  , readings will also vary from hot to cold eng , the big boyz calc is very handy for creating start point for  build , the main thing is that you see even readings ,front and rear , i have seen people say they were getting variation but it was due to not knowing how to use and test properly with a gauge , it may seem like a simple job , but there a correct procedure , nice to see readings close , i have 2 gauges and at 200 psi, one reads 195 , cheers

I'm in hydraulics and generally gauges have a 5-8 % error factor, you can get 2% but your talking big bucks. As stated the procedure for checking comp is the most important thing, and no more than 10% variation.