Just an update, I decided to go ahead with the 555 timer version as it has less parts and also requires no calibration. So I built half a dozen and put one in my remaining spare DCS-RC box in case one of the ones on the layout ever craps out.
Of the half dozen I built, I only had a 3% variation in the time interval that they trigger, well within the tolerance needed to generate a reliable watchdog signal. I didn't have one of the 1% resistors, they're trapped in COVID delays. That being the case, I simply hand selected from a batch of 5% resistors using a 1% resistor as a standard to match them using a Fluke bench meter. Most of the timing variation I see probably comes from the 5% capacitor, a .47uf 1% capacitor is price prohibitive, it cost more than all the other parts, including the PCB!
555 Timer Based Watchdog Generator Board
The three pin female socket mounts as illustrated in the three holes at the end of J2.
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