The VOIDRUNNER'S CODEX is LIVE! Explore new worlds, fight oppressive empires, fend off fearsome aliens, and wield deadly psionics with this comprehensive boxed set expansion for 5E and A5E!
Such a guide would potentially be useful for me too, as I am playing an arcana cleric of Mystra. One of her tenets is to create new spells, but I'm really not sure how to go about it.
I suspect the only way to do it is to come up with something that seems fair compared to other spells of that level, and playtest to make sure it is. I'm pretty sure that's how the designers did it.