I use a precision gauge, too. It's called a cheap tape measure.

Since bike chains (with the exception of skip-link chain and that one really oversized rare BMX size) are 1/2" pitch, it means that a new chain will measure 1/2" center-to-center between every two rivets. Chains are considered worn out at about 1% wear, since this is when the extra space between the rollers begins to cause serious damage to healthy cogs and rings. The standard procedure is to tension the chain (so it isn't drooping visibly) and measure one foot of chain, starting centered on a rivet. You should end up at or about another rivet a foot away, but you will probably be slightly short of it. By doing simple math with the difference, you can calculate how worn your chain is. 1% wear will show up as an extra 1/8" of length over those 24 rivets. An extra 1/16" over 24 rivets is about 0.5% wear and a good time to think about a new chain.

You can spend $15 on a fancy laser cut chain measuring doohickey, or you can just use a tape measure and get a more accurate reading for free (unless you're so deprived that you can't beg borrow or steal a simple steel tape measure). Those chain checkers only measure about 6 inches of chain. With a tape measure, you can sample over a longer distance to get a more precise reading, which is an improvement. Did I mention it's free?