Probably 29. You have to allow for the spring to stretch out when weight is on it, plus, you want the shackles to angle in, even when some weight is on it.

To put the new spring on, use some big clamps, remove the center bolt (carefully) install only the main leaf, then reassemble the rest of the leaves onto the shackled main leaf. Otherwise it's a bear to stretch the complete spring that far an get the shackles in.

Don