I don't know where you got your information, but after thinking about 13t being too small for the 520 chain, I did a little research. I used to have a Honda 250R three wheeler and I just looked up the chain and sprockets for it. From the factory, it came with a 13t front sprocket and a 520 chain. And the 200 three wheelers came with an 11t and 520 chain. And we used to stock and sell a 10t for them as well.
We raced my old 250R and with mods it would run 75mph. In the 80s we ran many, many 10t and 11t front sprockets on 520 chains in some very severe conditions and often with limited maintenance that probably bordered on neglect. If I can find a 13t for my bike, I will try it and report back the results.
And I have a calculator next to my computer. I know it's old fashioned, but I am old and can calculate the ratio changes with it quicker than I can go to another web site and put in the info there.
Just to make clear - I never said a 13 sprocket would not work. What I'm saying it will have excess wear and tear on the chain. Running racebikes with all kinds of odd sprocket combos is completely different than running long miles on the street. A 13/40 sprocket combo has excessively more front tooth contact and due the smaller radius a lot more driveline loss than a 14/43. I've never raced dirt, but done circuit racing and it's clearly measureable on the dyno.
Also what is the service interval of a 250R? A few tens of operating hours? Maintenance is not an issue on a racebike, where despite the dusty environment mileage is minimal, and bikes are maintained often - ateast due the different gearing needs for different tracks. I'm doing long distances on my 500x, and with proper maintenance can get about 30k out of a chain set running 15/41 and 35k on 17/43. Might not be relevant for you or the rebel with it's somewhat less travel oriented seating - but this is important for me.
A quick internet search gave me that JTR makes 43 rear sprockets cor my application (dunno if the rebel has the same hub dimensions) part number is jtr 245/3.43. I bet your calculator doesn't do that. (Pun inteneded) BTW, if you'd check gearingcommander once, you'd find it is a very useful tool which can calculate a lot more things at once than what you could do only in a much longer time with your calculator.
As a sidenote. Not to measure our private parts, but to establish the baseline. Surprisingly I can use a calculator too. In fact, between my MSC in engineering, MBA, and currently pursued PHD in applied mathematics, I can probably calculate even moderately difficult things faster without a calculator than most people can do with it. But I still use tools which are very useful to automate repetitive tasks and can calculate many aspects of things at once. You don't have to enter any information to gearingcommander, just selsct the rebel500 once, and it will start next time with it's parameters. And it calculates all the differences in three defined gearings, top speed or given speed in all gears, sprockets distances, same tooth same link occurences, and many other things at once. I have no affiliation with the developer, but I find it very useful.