well drainage depends upon a lot of things. like Permeability of the reservoir (heterogeneity of the reservoir - lithologically), drive mechanism, API gravity of oil. If the permeablity of the reservoir is on the higher side then its better to space out your wells rather than drilling wells close to one another and ending up with the production interference. Also in case of water drive reservoirs its generally better to optimise you well spacing so that the oil production doesnt leave too much of left out oil due to coning in between two producers.
And yes well spacing is generally calculated from well test data. and well spacing can be different in a single reservoir depending upon the heterogeneity of the same.
Well spacing studies need to done on priority while generating a Field Development Plan with the help of well test data available and the geological model to optimize ultimate recovery of any oil / gas field, and data from any new well drilled should be used to update your geological model and simulate the same. Such studies help a lot while planning Water injectors / gas injectors and suitable positions for oil producers.

Hope it helped .