I know this is a really late response, but my FS said that success rates for a FET cycle were in the order of 30% (compared with 40% for a fresh cycle).

Keen - I've heard similar to what you said. There was also an article I read in a scientific journal that said the difference between putting back 1 or 2 embys was only about 1-2% (for example, 28% BFP rate for a 1-emby FET cycle and 30% BFP rate for a 2-emby cycle), rather than the success rates being additive IYKWIM. There is obviously a higher chance of multiple with a 2-emby cycle, but the actual success rate overall doesn't really rise much whether 1 or 2 embys are put back.

Honestly, it's hard to get my head around it ... when I eventally use my frozen emby's, the logical/rational part of my brain is going to want to have 2 emby's put back each time to give myself a better chance ... even though it seems the percentages don't support that.