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I get it on the constant error rate.  I'm not sure that the error rate of the average cheap digital watch is as constant as is widely believed particularly when the watches are not stored at a constant temperature.  That approach requires regular checks.  Maybe having a ship's clock and a HAQ watch that are both accurate enough to use without rating would be a better answer given that they are available, compact, and low cost.  I'm thinkout out loud, I guess.

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See my other post of today. If you want to improve the rate of your watch you need to keep the watches at a constant temperature. The usual practice is to keep the time keeping element , the crystal, in an "oven" that can be maintained at a higher temperature than ambient. But it works the other way too. You can keep the crystal is an icebox which also maintains a very constant temperature. Because of the way the crystal responds to temperature, if it is kept below its designed temperature it will run slow so all you have to do is "rate" the watch in your icebox and then keep it in there and compare it with your "hack watch" prior to taking observations. We had a discussion many years ago about this, somewhere in the archives.

gl