Well, let me try it this way then.

1:49 says a ratio of 1 part concentrate:49 parts water. This makes 50 parts

Or 1+49 says 1 part concentrate plus 49 parts water. This also makes 50 parts.

So, I come out with both meaning the same to me and that is the way I was taught it as an undergraduate, a graduate and also as an engineer at Kodak.

By no means can I make 1+49 equal to 1:50. If you say 1 in 50 this becomes ambiguous. You see, you never talk about ratio of one to the total, it is ration of one to another.

A ratio represents, simply, for every amount of one thing, how much there is of another thing (NOT THE TOTAL). For example, supposing one has 10 parts of water for every part of developer, the ratio of developer:water would be 1:10 and the ratio of water:developer would be 10:1. The total number of parts would be 11. This would also be a 1+10 solution.