You probably can't go past 13-29 or 13-30 with Campagnolo 10 speed. It's not compatible with the spacing of 10 speed Shimano, for example. (11 speed Campagnolo has the exact spacing of 11 speed Shimano/SRAM, so they work interchangeably, but it would need a freehub swap for the different spline pattern.)
The percentage jump between gear shifts depends on the ratio of the two cog's teeth, but 8-10% per shift is often typical.
Compared to a 25 lowest gear:
29 is 16% easier (just divide the two: 29/25 = 1.16)
30 is 20%
32 is 28%
34 is 36% easier.
With my 11-speed 50/34 front and 11-32 rear, I can climb steeper hills while still staying seated. I'll never switch back to 11-28. With the 11-32, my yearly elevation is higher, even with somewhat fewer miles.