Hi, please first read the related questions on the forum regarding the batch size and number of slots, as they explain their meaning and how to set them. Relation between ring dimension, batch size, num slots and length, Ressources for Batching, OpenFHE SetSlots method: what is the usage scenario?, Relations between batch_size, ring dimension and array of values, How to do dynamic batch packaging of plaintext？
Why would the results be imprecise if you pad your vectors with zeroes up to the next power of two? As long as you correctly write the functionality on only the slots of interest, you should get the correct result. If garbage values get in the slots after the slots of interest, you can always zero them out by multiplying by a plaintext mask, e.g., [1,1,1,1,1,0,0,0].