Did I get the 241 deal right?

I'm afraid not.

There are no trains from Paddington to Waterloo. Both originate trains to Windsor on different routes, but the only connection between the two stations in London is by tube, not train.

Your train ticket has to show that you used it (or could have used it) to reach the attraction to be eligible for the Days Out promotion. Clear instructions are on the website, and answers about which tickets are valid are in the FAQs.

You would be using a ticket to leave London to travel to Windsor, then return to London. You need to show that your trip originated in Windsor and are traveling to London to visit the London Eye. Your outbound ticket will say OUT on it and your return will say RETURN on it so the direction of travel is obvious.

To go to Windsor you leave from Paddington station to Windsor (no Waterloo), you will change trains in Slough, just changing platforms, it's a 35 minute train ride. When you arrive in Windsor and exit the train go to your left, walk up to the street, the castle is right there across the street. While in Windsor you should visit Windsor Great Park. After you tour the castle you will exit thru King Henry Viii gate, turn to your right and walk down to Highn Street, turn left and follow the street down to the park. As you walk thru the gates and then up to your left is gates to the castle, from there you have an excellent view of the royal residence part of the castle. The Long Walk (the road you see that enters the castle) is the private road for the royal family to use to enter and leave the castle.

Thank you. Yes, I meant to type Windsor not Waterloo. My error. Thank you for the clarification on the train.

You could travel to Windsor & Eton Central from Paddington on a single, buy a day return from Windsor & Eton Riverside to Waterloo for the way back, show both OUT and RETURN parts at the London Eye (make sure you don't pass the ticket through the barrier at Waterloo) Whether this saves anything for you will depend on how much of a discount you would get from the London Eye. returns are only £1.80 more than a single which may make this possible.

I would save 24.95GBP at the London Eye.

Day return tickets from and back to London are valid for 2-4-1- offers on the day of travel, after your return to London. We visited the Courtauld Museum this way after taking the train to Kew.

https://www.daysoutguide.co.uk/faqs

The problem for you may be that Paddington is considered "too remote" for the discount to apply to London Eye. It is not on the list of nearby stations. Whether this makes a difference, I have no idea, but Nigel appears to be saying it does not qualify, and I trust his advice completely.

Maybe consider a visit to Kew, by rail from and back to Waterloo, instead? It is first on the list of nearby stations.

https://www.daysoutguide.co.uk/london-eye

As it says in their help area

If you are making a one way journey or your ticket is valid for one
day, you can use your National Rail tickets and offer vouchers only on
the day of travel.

If you are making a return journey, you can use your National Rail
tickets and offer vouchers any day on or between your outward and
return journeys. For example, if you arrived on Thursday and will
return on Sunday, your tickets will be considered valid for the offers
on Thursday, Friday, Saturday and Sunday.

All tickets should show that you have travelled to the attraction by
train, whether that be ‘London Terminals’ for 2FOR1 London
attractions, or the train station nearest to the attraction itself.
Tickets that have a destination too far from the attractions location
may be denied by attraction staff. Details of the attractions nearest
station can be found on the attraction’s listing page.

I don't know if Paddington is too far, but it likely is, but the question is that the tickets are leading away from London, not towards.

A single to Waterloo should work on the same day, or a return to London Terminals, but not a single to Paddington, or a return from Paddington.

You may get somebody who doesn't care or looks the other way, but likely you wouldn't.

As I have pointed out recently there is increased awareness of the way that the offer is being used. I do agree with Nigel that a single to Waterloo from Windsor and Eton Riverside alone does seem good enough to satisfy the current written terms, although these could well change again as it is tightened up. Note that the two Windsor stations are not considered the same destination - even the price differs, so a return from one London terminal is not valid on the other route.