Look at the ITF junior system of rankings from the ITF website:
A player's ranking is calculated using the best six singles results
plus one quarter (25%) of the best six doubles results.
Best 6 Singles results = 950
Best 6 Doubles results = 880
One quarter of 880 = 220
950 + 220 = 1170 = Combined Ranking points total
I think a quarter is a bit high. perhaps 15-20% is fair but with that added incentive and the supertiebreak that now constitutes the third set in a doubles match, we should see many more players especially top players playing. look at the mad rush that's on now to garner points for the season ending shanghai masters.
imagine federer/nadal playing against safin/roddick