Considering you may be stuck using what they supply, you need to know that the thickness is suitable for the loads. I frequently have decided that based, not on a theory, but on a field test of a typical, or marginally as good as the average area. What we do is prepare a typical area with a tapered varying thickness of the base material. Place it and compact it. the we take a fully loaded dump truck and drive it on the test area, sometimes back and forth, observing deflection under the effect of the heavier axles. Also look at ruts developed and generally OK a rut not more than one inch deep. Then we take the thickness of the area that "passes" and that is used for the whole job (generally used for commercial parking lots in these cases.)
In your case, you may have some weaker subgrade places where a greater thickness may be needed. I'd also be prepared to add base where I see the traffic is deflecting too much and there may be bad rutting.
You may even decide to undercut obviously bad areas and replace that thickness with base, adding above that the "design' thickness base.
If there is going to be a lot of repetitive heavy traffic, you may decide to go a little thicker than the "test" area "design". Repeated loads on some areas (usually silty soils, saturated) will cause a build up of pore pressures, leaving a much weaker subgrade result. Wait until the next day and all is OK, until repeated loads come again.
In some cases the added "repair" base material is a much coarser breaker-run quarry rock to soak up the softer base material.
Before doing the test, see if you can just add a layer of that coarser material to see if that thickness can be used to salvage what yu now have, instead of more of the same (failed base type).
One one job I was on the coarser stuff was reject bricks from a brick yard. However, that got stuck between the duals of the trucks.
Be sure the base roadway is sufficiently wide that shear failures off to the side don't occur.
