It's a linear relationship, so take the time needed for a block from the solochance site and ratio that with the time you run your miner vs. its best share and see if you're "lucky" during the time you are calculating this. (Vroob)
It's not, the term "best share" is misunderstood and this is probably how you came up with this "solution" which I have to sadly inform you that it is wrong.
Your miner's first submitted share can be diff-1, or in plain English the closest to hitting a block without solving it, and then it mines for twice the "projected" time and still never finds a block, on the other hand, your miner can mine for a year with the best share not being remotely close to a quarter of the difficulty at any given time, and then out of nowhere - it hits 2 blocks in a row!!. There is absolutely no linear relationship between your actual luck and the best share you submit, in fact, there is no relationship between your luck in hitting the first block and your luck in the second block.
Remember that POW has no record or memory of your previous shares, whether it's 1 or diff-1 it doesn't matter, it's like rolling a dice, every single roll is completely independent of both the past and future once.