Hard fact from Apr seniority list, between seniority numbers 900 and 1000 there are 27 FOs. 15 of these FOs are on a base, which suggests that they are unlikely to have ever started a command course. Thus, from this single snapshot a possible failure rate of 12%. I accept there are going to be exceptions to my simple assumptions, but I'm sure that this is roughly the sort of accurate figure and does include second time round passes.