Just a small hint here (e.g. how I solved it at one point, and the solution was good enough for me) - when you use enclosed objects, it might be enough to just render back faces into shadow map - that way you would get shadow acne only on faces that are facing away from light (but obviously they won't be hit by direct light from that light).

As for your original question, each shadow caster in my engine has its own offset (which allows you to set these values per-shadow-caster). Within WYSIWYG-like editor, it is pretty enough; so in my case you can determine the value by try & fail.

The value calculated from the following - each shadow caster has range (far - near), and precision (determined by your depth map precision, beware using just floating point value means your precision is not linear on whole scale ~ it will be a function based on the actual distance from your light, e.g. not constant), you can determine the following - if you increase your stored depth value by smallest value you can, how many units in space you "walk away" from light. Based upon this, you can determine the offset to apply on real depth value you are comparing to projected depth map.