Testing at the hose bibb..likely fed on a 1/2" line a long way from the meter, gave you the expected results. By the way, 70 PSI was the static pressure, but I suspect the dynamic pressure was substantially lower. This would NOT be a good place to tap off for the sprinkler.
If you tap near the meter, and ran 1 1/4" pipe from there to the manifold, you could get your 25 GPM with very little pressure loss and an acceptible water velocity of around 5 FPS.
By the way, at any supply store or even the depots, you can by a testing manifold rig which measures GPM and dynamic PSI. This could give you definitive results before you finalize your design. http://www.toro.com/sprinklers/irrmi...ges/53351.html