user1--you havent said what kind of riding you'll be doing, ie always unloaded or sometimes with more weight on the bike? How much does your bike weigh if its mostly ridden unloaded?
I ask because when I changed my granny to a 26 from the 30, it works great with a 12-27 on the rear if my bike is unloaded or lightly loaded. If more weight is going on the bike, I change it to a 11-32 and that covers it well with 30-40lbs on the bike.
I mention this because if you ride primarily unloaded, I wouldnt put the 24t on, a 26t would be better (imo) because the shift down to the 26 from the 40 won't be quite as big a drop as a 24. If you've ridden with a 30t granny before, you know the downshift from mid chainring (40 or 39) to the 30 granny is quite smooth, often just needing a one gear upshift at the rear (or none at all) but with the 26t, I tend to have to go up two gears.
I rode a bike with a 50/40/24 for years, and the 16tooth diff means you really do have to shift up 3 gears often at the back when you go into the granny--sure you get lower gears, and for touring thats ok, but I'd say the 26 might be nicer to live with for you.
do a gearing chart of a 30t vs a 26t vs a 24t and at least you'll see some gear inch numbers that will give you an idea of where they line up and what you'd expect going into the granny.
Oh, no mention of what fd you have, my Tiagra fd handled the drop from 30t to 26t with no issues, no adjustments required and works perfectly, but might not be the case of your fd.
and a chain catcher doohickey would be good too, they are cheap insurance from dropping a chain going to the granny with a now larger jump than before.