The following map was generated with Glideplan software and shows the necessary altitude to reach any of the airports that are at the center of the glide rings. This is based on a glide ratio of 1:27, attainable in still air with a 2nd generation glass ship such as a Discus or a DG 505, flown at a cruising airspeed of 80kt dry or 90kt wet. It also assumes an arrival altitude of 1,500 AGL above the target airport.
(Note: you should be able to get a considerably better glide ratio by flying at the best speed to fly but I’m using 1:27 because it reflects a more realistic cruising speed, and it provides an appropriate safety margin in case of moderate headwind or short stretches of moderate sink.)
IMPORTANT: the software does not account for terrain clearance! I.e., it is only indicative and must not be relied upon in mountainous terrain!
Here is link to the map: Boulder GlidePlan Map L/D 1:27
The map is very helpful for flight planning purposes in conjunction with the Colorado Soaring Area Map which you can find on this page. The GlidePlan Map can also be printed and left in the cockpit as it is very easy to see which airports are within glide range.
If you want to easily create your own custom maps based on airports you are most comfortable with and with your own settings for L/D and arrival altitude you can purchase your own copy of Glideplan.