I found a interesting snippet about clover %s in pasture last night. The
source is organic pastoral farming however many of the things discussed have
excellent transferability to lawns. The particular discussion was about
building herbal lays.

"The first consideration is to rebuild the humus in our soils to supply
carbon. The most important plant to get into your pasture mix is white
clover. This is the powerhouse plant for biologically based pastures.
Fortunately most farmer pastures do have clover, often in conjunction with
ryegrass, but usually not in high enough density being around 20% of the
sward. To optimise productivity clover content needs to be a minimum of 30%
of the sward, aiming for 50%."
http://www.biodynamic.org.nz/guides/ch3env_app.pdf (page 119 in this guide)
http://www.biodynamic.org.nz/guides/intro_ch1.pdf (an accompanying guide,
very very interesting)

I have a good covering of clover in my lawn. It has developed since I
stopped using broad leaf sprays and cut back on nitrogen heavy fertilisers.
The clover supplies much of my nitorgen needs now. It has packed quite
densely in some areas but seems to complement the grasses nicely giving a
decent effect on the lawn. Cut out the sprays & the chemical fertilisers &
your lawn can look quite quite nice I have discovered.

rob