To get a perfect Rhyniognatha you must:

Use a creature with at least 900 drag weight for the best possible stats (Gigas, Carchs, and Brontos have over 900 drag weight)

A max level rhyniognatha is required to get full stats from the host creature (nothing lower than max level for your settings)

Feeding the rhyniognatha all 5 cravings will get maximum leveling

When all of these requirements are met when deciding the babies stats it is the same as if the host creature was breeding with the rhyniognatha that impregnated so even if all of the requirements are met there is still a chance that not all the desired stat points will be received by the baby

More Rhyniognatha Taming & KO Tips