Does the squat allow to estimate the 5WD at the barbell lunges?
No. It does not. Having a higher squat may allow you to have a higher lunge just by the fact that it makes you stronger. However, they are fundamentally different movements and as such, there is no set formula where you can guess what weight your lunge should be.
The strategy to find the appropriate weight is simple. Start with a weight that you can easily handle without issue. Do a set for the rep range you are targeting. Gauge how easy it was. Increase the weight slightly higher and do another set. After two or three sets, you'll find a weight where it actually is a struggle. Stick with that continuing forward.
Assistance exercises typically aren't used in the 5 rep range as it is. They are meant to correct imbalances (for example if one leg is much stronger than the other). As such, it would be better to do them at a lower weight but higher volume compared to compound lifts like squat.