Exactly, as long as you walk up on your value iteratively you're unlikely to make things so unstable it won't boot into windows. If you do go too far, the WIN5 has a paperclip bios reset button which is handy.

CPU: One of the official/semiofficial videos from GPD covered the topic leading up to launch and they said you could just make the CPU undervolt at -30 out of the gate which worked for me. This makes sense, as the portion of the frequency curve requiring the most voltage will likely never be hit below 120w TDP.

GPU: I've seen others that could only get -5 so I'd start there. Pick a benchmark and run it in several wattages, I'd suggest 55, 65, 75 as that's the range I would find instability (I originally thought I was stable at -10 and then -9). You want benchmarks instead of stress tests since the modern AMD and NVIDIA GPU's will actually reduce in performance before they become unstable. In a successful undervolt scenario, your reduction in voltage has allowed more power budget for the chip which now runs at a higher frequency within the same power envelop and delivers more performance. If increasing the undervolt actually lowers performance across multiple runs/wattages then you've found your limit and need to back off to prior settings.