I may just order some of the 210 or 280 size thread.
I have never used floss so I was just wondering about what size thread it would be close to.
There are different types of floss and thread is not a very good substitute. As a general statement, floss is a very thick version of thread that allows more coverage of the hook with fewer thread wraps. But then it gets more complicated.
There are many different types of floss. Personally, I use 3 types more that anything else, 4 strand Rayon, single strand, and UNI-Stretch. For a beginner, I would avoid silk floss, not worth the extra effort or expense for most patterns.
I use 4 strand most often because I can vary the number of strands used and adjust the thickness of a floss body to produce a smoother finish << this is very important for tying some of the classic patterns, but it is not important at all for fishing flies. 4 strand is very versatile, but can be more challenging to use.
Single strand is very similar to a very thick thread and is easier to use that 4 strand. After discovering UNI-Stretch, I seldom use it anymore.
UNI-Stretch is a stretchable, more or less single strand floss. For fishing flies, it is quick and easy to use. This might be the material to try for your fly pattern.
Concerning 280 thread, while it looks like a rope compared to other sizes of tying threads, the spool I have looks to be only about half as thick as one strand of 4 strand floss. So, about 1/8 the size of single strand floss. You'd have to use a lot of it to replace floss.
Do you have a picture of this fly?