照旨意數目 Rain by the Written Number
大聖既有拔濟之心，容小龍回海點兵，煩大聖到天宮奏准，請一道降雨的聖旨，請水官放出龍來，我卻好照旨意數目下雨。」大圣既有拔济之心，容小龙回海点兵，烦大圣到天宫奏准，请一道降雨的圣旨，请水官放出龙来，我却好照旨意数目下雨。」
逐句解 Line by line
"Since the Great Sage has a mind to deliver them, let this small dragon go back to the sea and muster his troops, while the Great Sage takes the trouble to go up to the Palace of Heaven, obtain approval, and request an edict for rainfall. Then let the Water Officer release the dragons, and I shall be able to send down rain to the amount the edict specifies."
照旨意數目 — according to the number in the edict. This is the phrase to remember. Rainfall in this novel's heaven is not weather; it is an allocation, issued in writing, to a stated quantity. 點兵, to muster troops, and 水官, the Water Officer, complete the picture: a department with a roster and a controlling officer.
The dragon hands Pilgrim a workflow. Go up, get approval, obtain the edict, have the Water Officer release the dragons, and then rain will fall in the specified amount.
That last phrase is the one to hold on to. Rain here is a figure on a document. When the rain finally does come at the end of this chapter it arrives to a precise measurement, down to the last fraction — and the novel will state that figure twice, because the joke only lands if you have understood that somebody in heaven wrote it down.