I think that you're putting the cart before the horse. First find the gradient to the obstacle, and then limit the aircraft weight for the environmental conditions to ensure that that gradient can be achieved.
For your example of a 500 foot obstacle at 5000 M, the obstacle-clear gradient is 500 / (5000 / .3048) X 100 = 3.05%
Now, ensure from your aircraft performance data that a gradient of 3.05% or more is available at the proposed Takeoff Weight. If not possible, limit the weight until the aircraft is capable of a Net Gradient of 3.05%
NOTE - 1st Segment considerations not made in this simplified response.
Regards,
Old Smokey