I hardly use SP potions myself, and with just level 1 Zemyna + bonfire, I can fill up my SP in less than a minute without any problems. Zemyna per level does not increase its regeneration potential, instead it only increases SP cost reductions further at a pretty low amount. If you prefer regenerating your SP, then level 1 is the ideal level you only need.
It’s really not worth to invest any more points into the skill (see all the available build guides that feature Diev and they all say the same thing) and I highly recommend that you don’t try it out. Unless you’re willing to use a skill reset potion that you have lying around or you’re willing to buy one in the off-chance that you are disappointed with the results, I wouldn’t bet on something that has been tried and tested before.
Don’t forget that they recently added SP potions rewards in quests and while leveling as a C2 Cleric C2 Diev, I barely had any SP problems thanks to the huge amount of SP potions given away (if anything I got annoyed because those potions would fill up my total weight pretty fast).
I know that I might be extremely rude for arguing about your choices, but I’m only doing this so you make the best possible build you can. I’m not going to stop you in making your choices, but I do hope you take this advice in mind while you are doing so.