You have the question backwards.
It should be how fast you should be traveling so that you have enough space for stopping within your reaction time.
It is typically more relevant to discuss the term Stopping Distance which also includes the reaction time. The reaction time for situations you are not prepared for (such as most situations in traffic) is approximately 1.0 second on average.
For 40 kph, stopping distance is about 20 meters (Breaking distance is 8.0 meters and Reaction time-delay distance 12 meters).
The calculation is an estimate based upon empirical studies on normal road surface conditions. Thus, it will give you a rough estimate during normal, dry conditions but for wet, snowy, icy conditions these calculations do not apply at all.
According to the information at the links you provided, low-beam headlights on 80% of vehicles on the road may not provide adequate stopping distance at speeds above 40 miles per hour (65 kph - stopping distance = 45 meters) on unlit roadways, according to a study by AAA and the Automobile Club of Southern California’s Automotive Research Center. You erroneously quoted low-beam data as high-beam results.
From what I can gather, your decision to remain under 40kph should be well within the high-beam illumination provided by a modern commercial rental car which must be equipped and maintained according to local standards. You shouldn't need any supplementary non-standard lighting.
The simplistic solution is relatively easy to accommodate:
SLOW DOWN. Do not exceed the speed that will send you beyond the illuminated stretch of road.
More light throw won't make that much difference on poor roads anyway where damage from uneven road surfaces and unexpected obstructions make whatever gains you achieve marginal.