General advice (unless you have some high-performance wing) is not to aim for Marshall launch when leaving Crestline from its ridge height. Instead aim for Regionals (or halfway down the West-most spine down from Cloud peak), so you can go around to the West if needed, instead of getting trapped behind Marshall.
One has to expect that the first half of the glide from Crestline towards the front range (Marshall or Cloud) will feel good (not too much altitude loss). But as you get into the last half, you enter the sinking airflow (not necessarily rotor) that happens downwind of the Marshall-Cloud ridge line, and at the same time realize it will now be challenging to go around to the West.
Not a bad plan anyway to first aim for Regionals (or just West of Cloud peak), as in addition to being a shorter route to safety, you are likely to find lift in that area that you can then use to complete your “L” shape path towards Marshall (if needed).
How much height over Crestline ridge makes it safe to head directly to Marshall? Depends on headwind and expected conditions, but if you are 1000′ over the Crestline launch, your odds are good.