Hello Bob C.!
Bob McD is on the money. In other postings, particularly an article in The Academy, I've mentioned clearance, which is what you're talking about.
With 24" radius, you should have all the bases covered. On my last layout, I had 24" and my 4-8-4's and my Rivarossi Challenger made the curves just fine, but let me ask you this....
The plan you're proceeding with calls for 24" radius as a minimum. Great. Now, with the space you have, how large can you go? Can you go 30"? If you have the space, why not? Working with flex track, you can go as wide as space will permit. Can you increase the radius and adapt it to the plan? You will always be glad you did. What are you operating? Full sized passenger cars? 85' auto parts cars? If this is what you want, go for the maximum radius possible. Like a Lexus, once you've seen a train ease into a 54" radius curve, there'll be no going back!
Finally, when you lay paralleling curves, make sure that your tracks are two inches apart from the center. This will assure you clearance of the longest equipment currently available on the market, provided you go no smaller than 24" on the curve.
George.