Forgive the writing, please.
Begin with the group designation of the hex where the cleric has gained permission to build. This refers to the pre-existing infrastructure of the area. Hex groups six miles in diameter are rated in value from I to VIII (using Roman numerals), where I possesses the highest level of infrastructure and VIII possesses no infrastructure at all. Hex groups with designations of VIII have no inhabitants. Those with VI or VII have few inhabitants; those with I or II have many.
Note, however, that the cleric does not draw from the population of the entire 6-mile hex group. Each group is divided into seven 2-mile hexes - it is the inhabitants of this local 2-mile hex that is used to determine the cleric's potential congregation.
The cleric's charisma is expressed as a percentage multiplied by the infrastructure designation. The DM will be able to calculate the total number of inhabitants in the hex. Where the population is very high (densest levels will be more than 20,000 persons per 2-mile hex), there will be competition from other temples that will lower the cleric's draw. Each 1,000 population will reduce the cleric's charisma by 1 point.
Thus, a cleric with a 15 wisdom, building a temple in a hex group designated V would draw a base 75% of the inhabitants of the 2-mile diameter hex surrounding the temple. If, however, the total inhabitants numbered 3,000 persons, the cleric's charisma would be reduced to 12 and the calculated percentage reduced to 12 x 5 or 60%. If this number is reduced to zero, then the cleric's charisma and temple alone is not enough to draw worshippers in the area.
If it happens that the potential congregation is greater than 100% of the area's inhabitants, this will mean that additional worshippers are being drawn from other surrounding hexes, beyond the location of the temple.
Increasing the size of the temple can improve a cleric's charisma. Ornamentation will improve a cleric's charisma.
Where the temple itself is not be large enough to accommodate the whole potential congregation, clerics may rely upon a full congregation weekly. Where the reverse is true and the congregation is actually smaller than the temple, then efforts such as expanding the temple's size would be superfluous.
Note that the numbers above describes only the potential worshippers. To actually gain a congregation, a cleric must sacrifice 1 week in personal communication with the residents to build 10 weekly attendants. Thus, if a temple had a capacity of 200 worshippers, the cleric would need to spend 20 weeks total to bring that temple up to its full capacity weekly.
To retain a congregation, the cleric must deliver a service once per week. Each failure to appear will reduce the total congregation by 15% - whereupon more weeks of personal communication must be sacrificed winning these worshippers back. This pressure can only be relieved by obtaining a lesser cleric to jointly manage services - but this cleric too must also communicate with the locals, in effect building a parallel congregation using the same base temple, based upon that lesser cleric's charisma.
While the balance of weekly parishioners will offer tithes of 1 or 2 c.p. per week, or even none at all, the balance of wealthier worshippers will balance this so that the cleric may count on 2 s.p. per attending worshipper per week (a random die roll is suspended due to the potential of needing a hundred or more rolls). Moreover, if the cleric attends their temple for four weeks in succession, if the cleric succeeds in a charisma check then the cleric will receive a gift will be received of 20-80 g.p. Roll a d20 to determine the form: livestock (1-3), land (4-9), food for the poor (10-13), a contribution for expansion/ornamentation (cleric's decision) of the temple (14-15), a legacy in the form of coin (16), furniture (17), incense (18), a silversmithed object (19) or clothing (20).
If a cleric has gathered more than 100 worshippers and commits contractually to giving services no less than three times per month, the community/local lord will pay a stipend of 50 g.p. per month to the cleric. If the coin is accepted, the cleric will be asked occasionally to take actions which will pacify the local population (speaking out against violence, encouraging the peasants to know their place, feeding the poor, etc.).
If the temple exists in an area where the cleric's religion is part of the accepted social structure, the cleric will be asked to contribute 10% of the tithes received to superiors - this payment does not extend to gifts received nor the cleric's monthly stipend. If the cleric's religion is isolated from the community, this percentage of tithes need not be paid - but should the cleric wish to seek out the appropriate superiors and pay it anyway, there are benefits from this.
For example, a subordinate cleric may be requested from the religious establishment, to be assigned to the cleric's temple. This subordinate will act as a completely loyal follower, who will serve and defend the temple as able.