Such a sphere would have a radius of 92.832ft The range of fireball is 150ft, so you could safely cast it’s 100x magnitude counterpart.
Maybe I’m stupid but I don’t see how you could safely cast it. Even if you cast it all the way back at the edge of the range you still would be inside the radius.
If you cast it so that it’s point of origin is 150ft away from you, then it would expand both 93ft towards you and 93ft away from you. By expanding 93ft towards you, there’d still be a gap of 57ft between you and the edge of the fireball. The farthest point edge of the fireball would be 243ft away.
The radius is the size of the sphere as measured from the center of the explosion.
Maybe this analogy helps.
You throw a ball at a wall and it bounces back towards you. If the ball reaches you or passes you then you die.
You can throw a ball (the fireball) 150ft. At the end of your throw it hits a wall (center of explosion). The ball bounces back towards you 92.832ft (explosion radius/edge of explosion). The ball is now 57.168 ft away from you. Ball did not reach you therefore you live.
