The Sacred Drum has an odd area of effect. I tested its range by standing in the middle of a 100 tile farm. Depending on where you stand the shape changes. The effect is slightly larger than a 5x5 box and slightly off centred.

After a bit of testing, I figured out that the most consistent farming setup is to make a 5x5 farm.

Next you're going to want to stand in the middle tile but in the bottom 20% of it facing forward.

If you use the sacred drum here it'll consistently hit everything in the 5x5 area in one use of the ability.

It can splash out a couple tiles depending on where exactly you stand. Make use of that info how you will. In the screenshot, it sometimes hits the 2 trees planted to the right.