I need true always on mode for stopwatch (without rounding). how?

Hello timingst,

Thanks for reaching out to Apple Support Communities.

Let's check the following settings:

"How to turn the Always On feature on or off

1. Open the Settings app on your Apple Watch. To do this, press the Digital Crown to see the Home screen, then tap the Settings button .
2. Tap Display & Brightness.
3. Scroll down, then tap Always On.
4. Tap Always On to turn on or turn off the feature.

When your Apple Watch is in Low Power Mode, Always On isn't available.

"

Use the Always On feature with your Apple Watch - Apple Support

Need more help, Contact Apple for support and service

Cheers.

As far as I can tell that's not available. Of course, we here can't help you. But you can tell the people at Apple what you need here:

Feedback - Watch - Apple