Alright, so with the signing bonuses rolling in, let's do some math and figure out how the Astros are doing financially with the new system:
Total Bonus Pool: $11,200,000 to spread over 11 players.
Committed Bonuses: $5,150,000 (Correa, Phillips, Minor)
Other signees, at slot: $889,900 (Rodgers, Aplin, Bircher)
Total bonuses: $6,039,900
Recommended bonuses for unsigned players: $2,754,800
If we add up the total bonuses (again, we're having to guess at Rodgers, Aplin, and Bircher), both committed and recommended, we get $8,919,700 - or $2,280,300 under the allotment. So it's possible that the Astros could bump McCullers' offer up to $2.5m and still have $1,039,000 left to go after the prep unsignables (Ruiz, Virant, Hinojosa, Traver). Also, keep in mind that if the Astros do not sign Ruiz, they lose the $360,200 recommended for his draft position.
10th Rounder Joe Bircher was a senior (as is 7th Rounder Preston Tucker), so it's doubtful they signed right at slot, likely giving more wiggle room to the numbers.