Particles density instead of total number

I use Particles + Weight Painting for populating a vegetation (like trees) in my scenery.

The current Particle System needs to know the final particles count, but this workflow doesn't work for me well:

• I always know how dense my vegetation should be, but I never have an idea what number gives me the desired density, as it differs for each ground model (emitter) = need of guessing some numbers randomly which is often confusing, especially when ...
• I have to re-adjust this settings every time I weight-paint the model or the ground (emitter) size changes

For these reasons, I would appreciate an alternative parameter - the Density.

With Density = 1, the particle system would:

• generate 1 particle per 1 unit (i.e. square meter), so there would be 30 particles generated on an emitter of 30 square meters area
• it should also incorporate the weights, so if all the model has weight = 0, no particles will be generated
• 4 square meters painted by weight 1.0 = 4 particles
• 4 square meters painted by weight 0.5 = 2 particles
• and so on ...

I believe this could be huge time-saver for many people as it would lead into much more predictable and hassle-free workflow.