RTSS Scanline Sync is extremely difficult with modern games.
1. With capping turned off, you MUST have a framerate range above your refresh rate
2. You should use VSYNC OFF (easiest) with Scanline Sync.
3. You may need to limit RTSS Scanline Sync to older games
Reduce Your Refresh Rate To Cover Your Frametime Spikes
Your refresh rate for RTSS Scanline Sync must be below your 0.1% frametime!
1. Run uncapped VSYNC OFF
2. Measure your frametimes using RTSS
3. Find your worst 0.1% frametime spike (e.g. 10ms)
4. Now divide 1000 by the milliseconds of your biggest frame time spike (e.g. 1000/10ms = use ~100Hz)
5. Reduce your refresh rate to that number. A 10ms frametime spike means you should reduce your refresh rate to ~100Hz.
Reducing your frametime spikes to permit higher refresh rates
1. Reduce game detail
2. Upgrade GPU
3. Stick to older games for RTSS Scanline Sync
4. Some games will give you unavoidable framerate spikes
RTSS Scanline Sync only looks+feels amazing if you keep your entire framerate range above Hz
If you are unable to do this, prefer not to reduce refresh rate, or the compromises outweigh the benefits (refresh rate too low), then please consider using alternative methods such as
HOWTO: Low-Lag VSYNC ON
Prioritizing on visuals and consistent glassfloor latency (even if a bit higher than a lower-but-varying lag):
When combined with Scanline Sync, there are situations where reduced refresh rates can look massively better with strobed operation (e.g. ULMB), since perfect 100fps@100Hz ULMB looks CRT quality, while 110fps@120Hz looks jittery/stuttery/harsh/ugly strobing. Oh and if you use motion blur reduction such as ULMB, please don't forget to raise mouse DPI (1600dpi+) and lower in-game sensitivity since ULMB benefits from high-DPI.
HOWTO: Using ULMB Beautifully or Competitively.
Hopefully your ~0.1% or ~0.5% frametime spikes permit you to stay at 144Hz or higher, but RTSS Scanline Sync is much easier with older games like Quake, CS:GO, emulators, 60fps-limited console ports, and other games that are more amenable to avoiding frametime spikes that prevent RTSS Scanline Sync from looking amazing.
Rare frametime spike are okay, just try to minimize them
It's okay for your sub-0.1% or sub-0.5% frametime spikes to be worse (the stutter+tearing briefly appear 1 in 1000 frames for 0.1% spikes, and the stutter+tearing appear 5 out of 1000 frames for 0.5% spikes). Regardless, you must have more than 99% of your frame-rate / frame-time range above your refresh-rate / refresh-cycle-time,
if you want RTSS Scanline Sync to look beautiful
For more reading, see threads
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RTSS now has new automatic Low-Lag VSYNC ON (raster based)
(easiest)
For advanced readers wanting to get into more advanced RTSS Scanline Sync as a Quick Frame Transport mode
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Optional: Large vertical totals for low input lag
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Optional: Using RTSS Scanline Sync to create Quick Frame Transport.
(harder)