[Luce]

You're missing the physics on the reason for IRS having considerably stronger springs.

With a live axle, the springs move the same distance as the tires in. With an IRS, they're usually inclined and have leverage against by being mounted on the lower control arm between the frame joint and upright joint so they only move 60%-70% what the tire moves.

With the greatly reduced unsprung weight of an IRS, you need much less shock and can actually run softer (effective) spring rates than a live axle.

Back to the OP, springs are only $40-$50 each new, so it's not the end of the world if your first guess isn't spot on. I would start at 300 if it was my money.