If you read french : https://www.histoire-genealogie.com/Comment-estimer-l-importance-de-la
If not, here a short translation. "Parish registers that give us almost all the baptismal, marriage and burial certificates of a parish for any year. To approximate the population of a parish over a decade, a ten-year average of baptisms can be established. "Assuming that the birth rate was between 36 and 40 per thousand (which is very likely at least before 1750, and even long after that for the rural parishes), it is enough to assign the average birth rate of the coefficient 25 (corresponding to a rate of 40 per thousand) and the coefficient 28 (corresponding to a rate of 35.71 per thousand): the population figure is, with the maximum likelihood, within the range thus obtained." (d’après François Lebrun, Les registres paroissiaux et d’état civil in le Guide de l’histoire locale, Paris, Seuil, 1990).
Hope it could be helpful.