I have an old road bike with 53-39 rings and 13-23 cassette. Although it's generally flat in my area, I like to get to the top of these hills where they put the radio antennas (without dying). Max slope on these is 15%, average 8% and I suffer a lot.
I have spare cogs, rear mech is Shimano RX100 and thought I can fit a 26T in the cassette Which cog should I remove from my current setup (13-14-15-16-17-19-21-23)? I guess either the 16, the 21 or the 23?
I'm already assuming body fitness can be improved ;) Thanks!
Edit: here are the gear ratios http://gears.mtbcrosscountry.com/#700c/23I61I3909X700c/23I61I193X700c/23I61I15271X700c/23I61I15275
Report back (edit 2): I set 13-14-15-16-18-21-23-28 today. I just realized I have a 24 I could swap for the 23 to make a fairer gap, and I still have the 26 available. There is room for the 28T and didn't touch the B-screw. For now, it shifts fine standing, I have still to ride test it. I'll get back.
Conclusion - edit 3 :) Went for a small ride with climb included. Setup is still very close to what it was, introducing the 18 as Grigory said and leaving room for a life-saving 28 (thanks Criggie for the tip).
Shifting is good, I didn't notice much change, but as ChrisH and Criggie pointed out, it probably is a bit irregular on the new cogs. So I will eventually look for a complete cassette. Anyway, many thanks to all. I'll accept Grigory's answer as it addresses the original question with a better alternative to do what I intended.