I don't even pretend to fully understand how any of this works, I'm just going off what I've read and seen from some sharp minds.
I'm sure people come up with slightly different numbers, but Justin7 had said that BTCL by a point was equal to a certain %edge over the book. If the number goes from -4 to -3 then I believe you should put an additional wager on them minus 3 seeing as your edge is now greater.
I personally do not believe in playing lines, steam and all that is a dangerous toy to mess with in my mind.
But your 50% statement is wrong. Check out the "if you're betting the other side at pinnacle" thread in the think tank for an explanation from someone more advanced then me.
If DSI is offering San Francisco/Baltimore as a pk'em.
And
Pinny is offering Baltimore +150, and San Francisco -170.
The smartest bet to make is San Fran as a pk'em at DSI. And that would be more +EV than any arb involving a play at pinny. No matter which side you take at Pinny (assuming the line is accurate) you are paying vig on it. San Fran at -110 is beyond a vig free bet.
If you calculate the true odds based off of Pinny's lines, you will see that over a large sample size, the correct play is San Fran at DSI, without a hedge, because you LOSE value with your hedge. And if a wager post hedges expectation is above 50%, and it went down because of the hedge, then that means the initial expectation was greater than 50%.