Your allowed weight and span for a 2 x8 x12 2btr hem fir based on the info you have provided is 20 psf of dead load and 30 psf of live load with minimum bearing length of 0 68.
Attic wieght limit.
Make sure items are spaced so that the weight is evenly distributed over the chords.
If the attic has the potential to become a regular living space talk with a contractor about adding a fixed stairway.
Most trusses are designed for only 10 lbs psf live load for storage unless they are attic trusses like drooplug mentioned.
The dead load on the bottom chord of a truss varies with the weight of materials attached to it such as drywall on the ceiling.
That means you could lay a 2 x 4 ft.
All weight hanging from the truss has to be added to the existing dead load.
This is because the ladder is simply not strong enough to support any more weight and you are putting unreasonable pressure on the attic access ladders every time you break the weight limit.
If the total weight exceeds the load limit of the truss then the truss is compromised.
Of stuff on it.
Modern standards call for around 12 to 16 inches of insulation in the attic and your long term utility bills would probably appreciate it.
Observing the weight rating.
You ve added about 300 lbs already when you put 6 sheets of plywood up there.
If you want to use your attic on a regular basis or to store large items you may need to enlarge the access opening and install an access ladder or drop down stairs.
It is vital to observe the weight rating and make sure that you comply with it at all times.
Scrap of plywood over the chords and set about 40 lbs.
With an evenly distributed live load of 30 psf which the tables show the floor is able to support the total weight on the floor would be about 3 360 pounds.
Access to attic storage.
Is a safe estimate.
To simplify you can place 20 pounds of dead load per square foot on your subfloor as long it s supported mid span.
So if your ceiling is unfinished you have some excess carrying capacity up there.
Increasing the total weight on the floor to 4 480 pounds however results in a live load of 40 psf which is beyond the floor s load capacity.