You are doing well, although you failed to mentioned where you have seen/collected your alterna.
I agree w/ Lance's statement. It is probably impossible to put a ratio on the question you ask due to way too many variables.
For what it is worth, I have not missed a year in alterna habitat since 1987 and most years make 2 trips. My personal experiences and obervations show that the ratio you inquire about changes dramatically as you head west from Val Verde to the Bend. I have found 4 live and 1 DOR in Val Verde county in roughly 25 nights of searching. In the Bend I have found a mere 5 in over 200 nights and 2 of those were on one trip in 88 and 2 were on one pass one night in 94.
Most of finding what you are looking for is just being at the right place at the right time. Some are more fortunate/better at it than others.DB